A revolutionary exoplanet

In 1992, Aleksander Wolszczan and Dale Frail became the first astronomers to publicly announce that they had discovered the first planets outside the Solar System, orbiting the dense core of a dead star about 2,300 lightyears away. This event is considered to be the first definitive detection of exoplanets, a portmanteau of extrasolar planets. However, Michel Mayor and Didier Queloz were recognised today with one half of the 2019 Nobel Prize for physics for discovering an exoplanet three years after Wolszczan and Frail did. This might be confusing – but it becomes clear once you stop to consider the planet itself.

51 Pegasi b orbits a star named 51 Pegasi about 50 lightyears away from Earth. In 1995, Queloz and Mayor were studying the light and other radiation coming from the star when they noticed that it was wobbling ever so slightly. By measuring the star’s radial velocity and using an analytical technique called Doppler spectroscopy, Queloz and Mayor realised there was a planet orbiting it. Further observations indicated that the planet was a ‘hot Jupiter’, a giant planet with a surface temperature of ~1,000º C orbiting really close to the star.

In 2017, Dutch and American astronomers studied the planet in even greater detail. They found its atmosphere was 0.01% water (a significant amount), it weighed about half as much as Jupiter and orbited 51 Pegasi once every four days.

This was surprising. 51 Pegasi is a Sun-like star, meaning its brightness and colour are similar to the Sun’s. However, this ‘foreign’ system looked nothing like our own Solar System. It contained a giant planet much like Jupiter but which was a lot closer to its star than Mercury is to the Sun.

Astronomers were startled because their ideas of what a planetary system should look like was based on what the Solar System looked like: the Sun at the centre, four rocky planets in the inner system, followed by gas- and ice-giants and then a large, ringed debris field in the form of an outer asteroid belt. Many researchers even thought hot Jupiters couldn’t exist. But the 51 Pegasi system changed all that.

It was so different that Queloz and Mayor were first met with some skepticism, including questions about whether they’d misread the data and whether the wobble they’d seen was some quirk of the star itself. However, as time passed, astronomers only became more convinced that they indeed had an oddball system on their hands. David Gray had penned a paper in 1997 arguing that 51 Pegasi’s wobble could be understood without requiring a planet to orbit it. He published another paper in 1998 correcting himself and lending credence to Queloz’s and Mayor’s claim. The duo received bigger support by inspiring other astronomers to take another look at their data and check if they’d missed any telltale signs of a planet. In time, they would discover more hot Jupiters, also called pegasean planets, orbiting conventional stars.

Through the next decade, it would become increasingly clear that the oddball system was in fact the Solar System. To date, astronomers have confirmed the existence of over 4,100 exoplanets. None of them belong to planetary systems that look anything like our own. More specifically, the Solar System appears to be unique because it doesn’t have any planets really close to the Sun; doesn’t have any planets heavier than Earth but lighter than Neptune – an unusually large mass gap; and most of whose planets revolve in nearly circular orbits.

Obviously the discovery forced astronomers to rethink how the Solar System could have formed versus how typical exoplanetary systems form. For example, scientists were able to develop two competing models for how hot Jupiters could have come to be: either by forming farther away from the host star and then migrating inwards or by forming much closer to the star and just staying there. But as astronomers undertook more observations of stars in the universe, they realised the region closest to the star often doesn’t have enough material to clump together to form such large planets.

Simulations also suggest than when a Jupiter-sized planet migrates from 5 AU to 0.1 AU, its passage could make way for Earth-mass planets to later form in the star’s habitable zone. The implication is that planetary systems that have hot Jupiters could also harbour potentially life-bearing worlds.

But there might not be many such systems. It’s notable that fewer than 10% of exoplanets are known to be hot Jupiters (only seven of them have an orbital period of less than one Earth-day). They’re just more prominent in the news as well as in the scientific literature because astronomers think they’re more interesting objects of study, further attesting to the significance of 51 Pegasi b. But even in their low numbers, hot Jupiters have been raising questions.

For example, according to data obtained by the NASA Kepler space telescope, which looked for the fleeting shadows that planets passing in front of their stars cast on the starlight, only 0.3-0.5% of the stars it observed had hot Jupiters. But observations using the radial velocity method, which Queloz and Mayor had also used in 1995, indicated a prevalence of 1.2%. Jason Wright, an astronomer at the Pennsylvania State University, wrote in 2012 that this discrepancy signalled a potentially deeper mystery: “It seems that the radial velocity surveys, which probe nearby stars, are finding a ‘hot-Jupiter rich’ environment, while Kepler, probing much more distant stars, sees lots of planets but hardly any hot Jupiters. What is different about those more distant stars? … Just another exoplanet mystery to be solved…”.

All of this is the legacy of the discovery of 51 Pegasi b. And given the specific context in which it was discovered and how the knowledge of its existence transformed how we think about our planetary neighbourhoods and neighbourhoods in other parts of the universe, it might be fair to say the Nobel Prize for Queloz and Mayor is in recognition of their willingness to stand by their data, seeing a planet where others didn’t.

The Wire
October 8, 2019

How Venus could harbor life: supercritical carbon dioxide

The dark spot of Venus crossed our parent star in 2012. Pictured above during the occultation, the Sun was imaged in three colors of ultraviolet light by the Earth-orbiting Solar Dynamics Observatory.
The dark spot of Venus crossed our parent star in 2012. Pictured above during the occultation, the Sun was imaged in three colors of ultraviolet light by the Earth-orbiting Solar Dynamics Observatory. Image: NASA/SDO & the AIA, EVE, and HMI teams

A new study published in the online journal Life says a hotter, pressurized form of carbon dioxide could harbor life in a similar way water does on Earth. This is an interesting find, theoretical though it is, because it might obviate the need for water to be present for life to exist on other planets. In fact, of the more than 2,700 exoplanet candidates, more than 2,000 are massive enough to have such carbon dioxide present on their surface.

At about 305 kelvin and 73-times Earth’s atmospheric pressure, carbon dioxide becomes supercritical, a form of matter that exhibits the physical properties of both liquids and gases. Its properties are very different from what they usually are in its common state – in the same way highly pressurized water is acidic but normal water isn’t. Supercritical carbon dioxide is often used as a sterilization agent because it can deactivate microorganisms quickly at low temperatures.

As the study’s authors found, some enzymes were more stable in supercritical carbon dioxide because it contains no water. The anhydrous property also enables a “molecular memory” in the enzymes, when they ‘remember’ their acidity from previous reactions to guide the future construction of organic molecules more easily. Moreover, as stated in the paper,

… the surface tension in carbon dioxide is much lower than that of water, whereas the diffusivity of solutes in scCO2 is markedly higher [because of lower viscosity]. Thus, scCO2 can much easier penetrate [cell membranes] than subcritical fluids can.

The easiest way – no matter that it’s still difficult – to check if life could exist in supercritical carbon dioxide naturally is to check the oceans at about a kilometer’s depth, where pressures are sufficient to entertain pockets of supercritical fluids. As the authors write in their paper, supercritical carbon dioxide is less dense than water, so they could be trapped under rocky formations which in turn could be probed for signs of life.

A similarly accessible place to investigate would be at shallow depths below the surface of Venus. Carbon dioxide is abundant on Venus and the planet has the hottest surface in the Solar System. Its subsurface pressures could then harbor supercritical carbon dioxide. Dirk Schulze-Makuch, a coauthor of the paper and an astrobiologist at Washington State University, notes,

An interesting twist is that Venus was located in the habitable zone of our Solar System in its early history. [Him and his coworkers] suggested the presence of an early biosphere on the surface of this planet, before a run-away greenhouse effect made all life near the Venusian surface all but impossible.

The probability that Venus could once have harbored life is as strange as it is fascinating. In fact, if further studies indicate that supercritical carbon dioxide can play the role of a viable bio-organic solvent,  the implications will stretch far out into anywhere that a super-Earth or gas-giant is found. Because its reactions with complex organic molecules such as amines will not be the same as water’s, the life-forms supercritical carbon dioxide could harbor will be different – perhaps more primitive and/or short-lived. We don’t know yet.

This study continues a persistent trend among astrobiologists since the 1980s to imagine, and then rationalize, if and how life could take root in environments considered extreme on Earth. After the NASA Kepler space telescope launched in 2009 and, in only four years of observation, yielded almost 4,100 exoplanet candidates (more than a thousand confirmed as of now), astrobiologists began to acquire a better picture of the natural laboratories their hypotheses had at their disposal, as well as which hypotheses seemed more viable.

In August this year, Schulze-Makuch himself had another paper, in Science, that discussed how a lake of asphalt in Trinidad harbored life despite a very low water content (13.5%), and what this said about the possibilities of life on Saturn’s moon Titan, which exhibits a similar chemistry on its surface. The Science paper had cited another study from 2004. Titled ‘Is there a common chemical model for life in the universe?‘, it contained a pertinent paragraph about why the search for alien life is important as well as likely endless:

The universe of chemical possibilities is huge. For example, the number of different proteins 100 amino acids long, built from combinations of the natural 20 amino acids, is larger than the number of atoms in the cosmos. Life on Earth certainly did not have time to sample all possible sequences to find the best. What exists in modern Terran [i.e. Earth-bound] life must therefore reflect some contingencies, chance events in history that led to one choice over another, whether or not the choice was optimal.

ALMA telescope catches live planet-forming action for the first time

The ALMA telescope in Chile has, for the first time, observed a star system that might be in the early stages of planet formation. The picture has astronomers drooling over it because the study of the origins of planets has until now been limited to simulated computer models and observations of planets made after they formed.

ALMA image of the protoplanetary disc around HL Tauri.
ALMA image of the protoplanetary disc around HL Tauri. Image: ALMA (ESO/NAOJ/NRAO)

According to a statement put out by the European Southern Observatory (ESO), the observation was one of the first made with the ALMA, which opened in September 2014 for a ‘Long Baseline Campaign’ (ESO is the institution through which European countries fund the telescope). ALMA uses a technique called very-long baseline interferometry to achieve high resolutions that lets it observe objects hundreds of light-years away in fine detail. It makes these observations in the millimeter/sub-millimeter range of wavelengths; hence its name: Atacama Large Millimeter/sub-millimeter Array.

The image shows a disc of gas, dust and other debris orbiting the star HL Tauri, located about 450 light-years from Earth. A system like this is originally a large cloud of gas and dust. At some point, the cloud collapses under its own gravitation and starts to form a star, further accruing matter from the cloud and growing in size. The remaining matter in the cloud then settles into a disc formation over millions of years around the young star.

In the disc, the gas and dust continue to clump, this time into rocky lumps like planets and asteroids. This is why the disc is called a proto-planetary disc. As a planet forms and its gravitational pull gets stronger, it starts to clear a space in the disc of matter by either sucking it for itself or knocking it out. The gaps that are formed as a result are good indicators of planet formation.

According to the ESO statement, “HL Tauri’s disc appears much more developed than would be expected from the age of the system [less than 100,000 years]. Thus, the ALMA image also suggests that the planet-formation process may be faster than previously thought.”

An annotated image showing the protoplanetary disc surrounding the young star HL Tauri.
An annotated image showing the protoplanetary disc surrounding the young star HL Tauri. Image: ALMA (ESO/NAOJ/NRAO)

In the Solar System, similar gaps exist called Kirkwood gaps. They represent matter cleared by Jupiter, whose prodigious gravitational pull has been pushing and pulling the orbits of asteroids around the Sun into certain locations. In fact, Jupiter’s movement within the Solar System – first moving away, then toward, and then away once more from the Sun – has been used to explain why the material composition of some asteroids between Mars and Jupiter is similar to those of Kuiper Belt objects situated beyond the present orbit of Neptune. Jupiter’s migration mixed them up.

Similarly, the gaps forming around HL Tauri, though they may represent planetesimals, may not result in planets in the exact same orbits as they could move around under the influence of subsequent gravitational disruptions. They could acquire unexpectedly eccentric orbits if their star system comes too close to another, as was found in the nearby binary star system HK Tauri in July 2014. Or, the gaps are probably being emptied by the gravitational effects of an object in another gap.

However, astronomers think the presence of multiple gaps is likely evidence of planet formation more than anything else.

At the same time, the resolution in the image is 7 AU (little more than Jupiter’s distance from the Sun), which means the gaps are very large and represent stronger gravitational effects.

Astronomers will use this and other details as they continue their investigation into the HL Tauri system and how planets – at least planets in this system – form. The Long Baseline Campaign, which corresponds to the long-baseline configuration of the ALMA telescope that enabled this observation, will continue into December.

For planets, one thing leads to another

One of the biggest benefits of being a journalist is that you become aware of interesting things from various fields. As a science journalist, the ambit is narrowed but the interestingness, not at all. And one of the most interesting things I’ve come across is a relationship between the rate at which planets rotate – the equatorial rotation velocity – and their mass. It seems the lighter the planet, the lower its equatorial rotation velocity. This holds true for all the planets in the Solar System, as well as large asteroids and Kuiper belt objects such as Pluto. The plot below shows logarithm of planet mass (kg) on the x-axis and logarithm of equatorial rotation velocity (km/h) on the y-axis.

The blue line running through the points is a local regression fit and represents a statistical connection. However, there are four prominent outliers. They represent Mercury, Venus, Earth and Mars, the System’s rocky, inner planets. Their spin-mass correlation deviates from the normal because they are close to the Sun, whose gravitational pull exerts a tidal force on the planets that slows them down. Earth and Mars are also influenced by the gravitational effects of their moons. Anyway, I’ve already written at length about this fascinating connection. What I want to highlight here are more such connections.

And before that, a note: it’s probably obvious that they exist because the connection between mass and equatorial velocity could just as well be a connection between mass and a string of other properties that eventually influence the equatorial velocity. This thought was what led me to explore more connections.

Mass and rotation period

The rotation period of a planet describes the time taken for the planet to rotate once around its axis. If mass and equatorial velocity are related, then mass and rotation period can be related, too, if there is a connect between mass and planetary radius, which in turn implies there is a connect between radius and density, which in turn implies that planets that can get only so big and so dense before they become implausible, presumably – a conclusion borne out by a study released on May 26.

Image: A log-log plot between planetary mass and rotation period.

Density and rotation period

If a planet can only get so big before it becomes puffy, and its mass is related to its rotation period, then its density and rotation period must be connected, too. The chart below shows that that hypothesis is indeed borne out (it’s not my hypothesis, FYI): log(density) increases as log(rotation period) does, so denser planets rotate faster. However, the plot shows significant variation. How do you explain that?

Image: A log-log plot between density and rotation period.

Turn to the Solar System’s early days. The Sun has formed and there is a huge disk of gas, rocks, dust and other debris floating around it. It exerts a gravitational pull that draws heavier things in the disk toward itself. As it becomes more energetic, however, it exerts a radiation pressure that pushes lighter things in the disk away. Thus, the outermost areas of the disk are mostly dust while the inner areas are heavier and denser. It’s possible that this natural stratification could have created different bands of material in the disk of different densities. These different kinds of materials could have formed different kinds of planets, each now falling on a specific part of the log(density) v. log(rotation period) plot.

Mass and density

Of course mass and density are related: the relation is called volume. But what’s interesting is that the volume isn’t arbitrary among the Solar System’s planets. It rises and then dips, which means heavier planets are less dense and, thus, more voluminous. For example, Jupiter is the heaviest planet in the Solar System – it weighs as much as 317 Earths. Its volume is more than 1,300-times Earth’s. However, its density – 1.33 g/cm3 versus Earth’s 5.52 g/cm3 – shouldn’t be surprising because it’s made of mostly hydrogen and helium. Which is just what we deduced based on the mass-period connection.

Image: A log-log plot between mass and density.

One way to find out if there’s any merit in this exercise is to conduct a detailed study. Another way is to perform a regression analysis. I’m not smart enough to do that, but I’ll blog in detail about that when I am.

New category: Exoplanets

Using a network of telescopes scattered across the globe, including the Danish 1.5-m telescope at ESO La Silla (Chile), astronomers  discovered a new extrasolar planet significantly more Earth-like than any other planet found so far. The planet, which is only about 5 times as massive as the Earth, circles its parent star in about 10 years. It is the least massive exoplanet around an ordinary star detected so far and also the coolest. The planet most certainly has a rocky/icy surface. Its discovery marks a groundbreaking result in the search for planets that support life.
An artist’s impression of an icy exoplanet. Image: Wikimedia Commons

Of late, telescopes like Kepler, Spitzer and ALMA are revealing new things about exoplanets as much as they’re exposing how clueless we are about their origins. Unlike in the search for life, where our only precedents are terrestrial, the search for and study of exoplanet systems is aided by Kepler’s revelation of hundreds of them, in a variety of flavors. And the more of them we discover, the more it becomes evident that in many ways the Solar System is actually an outlier, and that subjecting our stellar neighborhood to more probing is only going to reveal so much about the incomparable world outside. Each discovery is an opportunity for astronomers to revisit some detail or other. Like in the Banach-Tarski paradox, a dismantled theory is pieced back together to predict different things with a twist or two. Needless to say, it’s an exciting time to be a planetary scientist – or an interested blogger. To keep track of developments, The Last Why has a new category called ‘Exoplanets’. The eight posts already filed in it are linked to below, with more to come. Enjoy the ride!

  1. Why do tilted/eccentric orbits form?
  2. Kepler data reveals a frost giant
  3. Looking for life? Look for pollution.
  4. Studying our primal horizons at the Kuiper belt
  5. What life on Earth tells us about life ‘elsewhere’
  6. Rocky exoplanets only get so big before they get gassy
  7. The secrets of how planets form
  8. Interactions between a planetary system and an FFP: A fuzzy approach

Kepler data reveals a frost giant

I’ve been most fascinated lately by studies of planet formation. Every small detail is like that one letter in the crossword you need to fill all the other boxes in, every discovery a cornerstone that holds together a unique piece of the universe. For example, using just the find that the exoplanet Beta Pictoris b has a very short day of eight hours, astronomers could speculate on how it was formed, what its density could be, and how heavy it could get over time. And it isn’t surprising if a similar tale awaits telling by Kepler 421b, an exoplanet some 1,000 ly from Earth toward the constellation Lyra. Its discovery was reported on July 17, a week ago. And its pièce de résistance is that it has a long year, i.e. orbital period, of 704 days.

Illustrating the transit technique. The technique applies only when the planet can be seen head on against the background of its star. Image: http://www2.ifa.hawaii.edu/

Image: Illustrating the transit technique. The Kepler telescope looks for the drop in brightness in its search for exoplanets. The technique applies only when the planet can be seen head on against the background of its star. Credit: http://www2.ifa.hawaii.edu/

To have such a long year, it must be orbiting pretty far from its star – Kepler 421 – which in turn should’ve made it hard to discover. The NASA Kepler space telescope spots exoplanets by looking for the dip in a star’s brightness as the planet moves in front of it, called a transit. Because of Kepler 421b’s high orbital period, it transited its central star only twice in the four years Kepler was looking at it. Together with its orbital eccentricity – i.e. how elliptic its orbit is – Kepler had only a 0.3% chance of spotting it on its way around the star. In fact, 421b has the longest year for any known exoplanet discovered using the transit technique. This means we need to start considering if the M.O. isn’t good enough to spot exoplanets with large orbital periods, a class of planets that astronomers have been looking for. On the other hand, now that 421b has been spotted and studied to some extent, astronomers can form impressions of its history and future.

The frost line

For starters, they were able to deduce the planet’s size based on how much starlight it blocked and the shape of its orbit from how much light it blocked during each full transit. The readings point to 421b being like Uranus, with radius four times Earth’s, density at least 5 g/cc, and an eccentric orbit. Being like Uranus also means a surface temperature of -90 degrees Celsius (183 kelvin). This is plausible because 421b is 1.2 times as far from its star as Earth is from the Sun, and its star is a dimmer orange dwarf.

These wintry conditions are found beyond a star’s frost line, an imaginary line marking the distance beyond which space is cold enough to cause hydrogen-based molecules to condense into icy grains. So planets orbiting beyond this distance are also icy. Kepler 421b is likely the first exoplanet astronomers have found (using the transit technique) orbiting a star beyond its frost line. In other words, this might be our first exoplanet that’s an ice giant – “might” because 421b hasn’t been independently observed yet.

Not surprisingly, the frost line also marks a more significant boundary in terms of planet formation. Though observations made by Kepler are starting to show that the Solar System is a surprisingly unique planetary system, it’s still the one we understand best and use to analogize what we finds in other worlds. Astronomers believe planets in the system formed out of a disk of matter surrounding a younger Sun. The inner Earth-like (telluric) planets formed when rocky matter started to clump together and “fall out” of this disk. The outer gaseous planets, beyond the frost line, formed when icy grains stuck together to form watery planetary embryos.

In this artist's conception, gas and dust-the raw materials for making planets-swirl around a young star. The planets in our solar system formed from a similar disk of gas and dust captured by our sun. Credit: NASA/JPL-Caltech

 

Image: In this artist’s conception, gas and dust-the raw materials for making planets-swirl around a young star. The planets in our solar system formed from a similar disk of gas and dust captured by our sun. Credit: NASA/JPL-Caltech

The prevailing belief is that planets take at least three million years to form. In the same period, the central star is also evolving – in this case, Kepler 421 is a K-class star becoming brighter – and the amount of material available in the protoplanetary disk is diminishing because planets are feeding off it. Consequently, the frost line is on the move. Calculations by the astronomers who discovered 421b find the exoplanet to be now where the system’s frost line might’ve been three million years ago.

The sedate giant

Right now, we’ve a lot of letters in the crossword. Piecing them together, we can learn the following:

  1. If a beyond-the-frost-line gas giant is as big as Uranus but not as big as Jupiter, it’s possible that not enough material was available when it started to form, rendering it a latecomer in the system
  2. The abundance of material required to form Jupiter-sized planets makes smaller worlds likelier than larger ones, and in fact implies worlds like 421b should be less unique than Kepler makes it seem (a 2013 study cited by the discoverers suggests that there might actually be a pile-up of planets transiting at the frost line of their stars)
  3. If the planet had to have formed behind its star’s frost line, and the frost line was three million years ago where the planet is now, the planet could be around three million years old – assuming it hasn’t moved around since forming
  4. 421b is very Uranus-like; if it has to be a rocky world, its mass has to be 60 times Earth’s, pointing at an improbably massive protoplanetary disk within one or two AU of a star – something we’re yet to find

#3 warrants a comparison with the Solar System’s history, especially Jupiter’s. Jupiter didn’t form where it is right now, having possibly moving toward and away from the Sun as a result of gravitational interactions with other planets that were forming. During its journeys, its own gravitational pull could’ve tugged on asteroid belts and other free-floating objects, pulling them out of one location and depositing them in another. Contrarily, 421b appears to have been far more sedate, probably not having moved at all due to its youth and isolation. If only it had moved inward, like Jupiter eventually did, its orbital period would’ve been shorter and Kepler would’ve have spotted it easier.

The confusion Jupiter might've caused during its journey through Middle Earth. Image: http://www.astro.washington.edu/courses/astro557/GrandTack2.pdf

Image: The confusion Jupiter might’ve caused during its journey through a nascent Solar System. Credit: http://www.astro.washington.edu/courses/astro557/GrandTack2.pdf

Another comparison can be made with Beta Pictoris b, the other exoplanet mentioned at the beginning of this piece, the one with the eight-hour-long days. Younger planets spin faster because they still have the angular momentum they acquired while accumulating mass before slowing down in time. Heavier planets also spin faster because they have more angular momentum to conserve. Similarly, we might be able to find out more about Kepler 421b’s past by uncovering its spin rate and getting a better estimate of its mass.

Anyway, a simple piecing together of facts and possibilities tells us – at least me – this much. Astronomers have one more awesome fact to take away: as the finders of 421b write in their pre-print paper, “the first member of this missing class of planets” has been found, and that means more astronomy to look forward to!

~

References

Discovery of a transiting planet near the snow line, Kipping et al, arXiv:1407.4807 (accepted in The Astrophysical Journal)

What life on Earth tells us about life ‘elsewhere’

Plumes of water seen erupting form the surface of Saturn's moon Enceladus. NASA/JPL-Caltech and Space Science Institute
Plumes of water seen erupting form the surface of Saturn’s moon Enceladus. NASA/JPL-Caltech and Space Science Institute

In 1950, the physicist Enrico Fermi asked a question not many could forget for a long time: “Where is everybody?” He was referring to the notion that, given the age and size of the universe, advanced civilizations ought to have arisen in many parts of it. But if they had, then where are their space probes and radio signals? In the 60 years since, we haven’t come any closer to answering Fermi, although many interesting explanations have cropped up. In this time, the the search for “Where” has encouraged with it a search for “What” as well.

What is life?

Humankind’s search for extra-terrestrial life is centered on the assumption – rather hope – that life can exist in a variety of conditions, and displays a justified humility in acknowledging we really have no idea what those conditions could be or where. Based on what we’ve found on Earth, water seems pretty important. As @UrbanAstroNYC tweeted,

And apart from water, pretty much everything else can vary. Temperatures could drop below the freezing point or cross to beyond the boiling point of water, the environment can be doused in ionizing radiation, the amount of light could dip to quasi-absolute darkness levels, acids and bases can run amok, and the concentration of gases may vary. We have reason to afford such existential glibness: consider this Wikipedia list of extremophiles, the living things that have adapted to extreme environments.

Nonetheless, we can’t help but wonder if the qualities of life on Earth can tell us something about what life anywhere else needs to take root- even if that means extrapolating based on the assumption that we’re looking for something carbon-based, and dependent on liquid water, some light, and oxygen and nitrogen in the atmosphere. Interestingly, even such a leashed approach can throw open a variety of possibilities.

“If liquid water and biologically available nitrogen are present, then phosphorus, potassium, sodium, sulfur and calcium might come next on a requirements list, as these are the next most abundant elements in bacteria,” writes Christopher McKay of the NASA Ames Research Center, California, in his new paper ‘Requirements and limits for life in the context of exoplanets’. It was published in Proceedings of the National Academy of Sciences on June 9.

Stuff of stars

McKay, an astro-geophysicist, takes a stepped approach to understanding the conditions life needs to exist. He bases his argument on one inescapable fact: that we know little to nothing about how life originated, but a lot about how, once it exists, it can or can’t thrive on Earth. Starting from that, the first step he devotes to understanding the requirements for life. In the second step, he analyzes the various extreme conditions life can then adapt to. Finally, he extrapolates his findings to arrive at some guidelines.

It’s undeniable that these guidelines will be insular or play a limited role in our search for extraterrestrial life. But such criticism can be partly ablated if you consider Carl Sagan’s famous words from his 1980 book Cosmos: “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.”

In 1991, RH Koch and RE Davies published a paper (titled ‘All the observed universe has contributed to life’) presenting evidence that “a standard 70 kg human  is always making about 7 3He, 600 40Ca, and 3,000 14N nuclei every second by radioactive decay of 3H, 40K, and 14C, respectively”. In other words, we’re not just made of starstuff, we’re also releasing starstuff! So it’s entirely plausible other forms of life out there – if they exist – could boast some if not many similarities to life on Earth.

To this end, McKay postulates a ‘checklist for habitability’on an exoplanet based on what we’ve found back home.

  • Temperature and state of water – Between -15° C and 122° C (at pressure greater than 0.01 atm)
  • Water availability – Few days per year of rain, fog or snow, or relative humidity more than 80%
  • Light and chemical energy sources
  • Ionizing radiation – As much as the bacterium Deinococcus radiodurans can withstand (this microbe is the world’s toughest extremophile according to the Guinness Book of World Records)
  • Nitrogen – Enough for fixation
  • Oxygen (as the molecule O2) – Over 0.01 atm needed to support complex life

McKay calls this list “a reasonable starting point in the search for life”. Its items show that together they make possible environmental conditions that sustain some forms of chemical bonding – and such a conclusion could inform our search for ‘exo-life’. Because we’re pretty clueless about the origins of life, it doesn’t mean we’ve to look for just these items on exoplanets but the sort of environment that these items’ counterparts could make possible. For example, despite the abundance of life-friendly ecosystems on Earth today, one way life could have originated in the first place is by meteorites having seeded the crust with the first microbes. And once seeded, the items on the checklist could have taken care of the rest.

Are you sure water is life?

Such otherworldly influences present yet more possibilities; all you need is another interstellar smuggler of life to crash into a conducive laboratory. Consider the saturnine moon Titan. While hydrocarbons – the principal constituents of terran life – on Earth are thought to have gassed up and out from the mantle since its formative years, Titan already boasts entire lakes of methane (CH4), a simple hydrocarbon. A 2004 paper by Steven Benner et al discusses the implications of this in detail, arguing that liquid methane could actually be a better medium than water for certain simple chemical reactions that are the precursors of life to occur in.

Another Solar System candidate that shows signs of habitability is Titan’s peer Enceladus. In April this year, teams of scientists studying data from the Cassini space probe said there was evidence that Enceladus hosts a giant reservoir of liquid water 10 km deep under an extensive ice shell some 30-40 km thick. Moreover, Cassini flybys since 2005 had shown that the moon had an atmosphere of 91% water vapor, 3-4% each of nitrogen and carbon dioxide, and the rest of methane.

These examples in our Solar System reveal how the conditions necessary for life are possible not just in the Goldilocks zone because life can occur in a variety of environments as long some simpler conditions are met. The abstract of the paper by Benner et al sums this up nicely:

A review of organic chemistry suggests that life, a chemical system capable of Darwinian evolution, may exist in a wide range of environments. These include non-aqueous solvent systems at low temperatures, or even supercritical dihydrogen– helium mixtures. The only absolute requirements may be a thermodynamic disequilibrium and temperatures consistent with chemical bonding.

As humans, we enjoy the benefits of some or many of these conditions – although we know what we do only on the basis of what we’ve observed in nature, not because some theory or formula tells us what’s possible or not. Such is the amount of diversity of life on Earth, and that should tell us something about how far from clued-in we are to understanding what other forms of life could be out there. In the meantime, as the search for extra-terrestrial life and intelligence goes on, let’s not fixate on the pessimism of Fermi’s words and instead remember the hope in Sagan’s (and keep an eye on McKay’s checklist).

Rocky exoplanets only get so big before they get gassy

By the time the NASA Kepler mission failed in 2013, it had gathered evidence that there were at least 962 exoplanets in 76 stellar systems, not to mention the final word is awaited on 2,900 more. In the four years it had operated it far surpassed its envisioned science goals. The 12 gigabytes of data it had transmitted home contained a wealth of information on different kinds of planets, big and small, hot and cold, orbiting a similar variety of stars.

Sifting through it, scientists have found many insightful patterns, many of which evade a scientific explanation and keep the cosmos as wonderful as it has been. In the most recent instance of this, astronomers from Harvard, Berkeley and Honolulu have unearthed a connection between some exoplanets’ size, density and prevalence.

They have found that most exoplanets with radii 1.5 times more than Earth’s are not rocky. Around or below this cut-off, they were rocky and could hypothetically support human life. Larger exoplanets – analogous to Neptune and heavier – have rocky cores surrounded by thick gaseous envelopes with atmospheric pressures too high for human survival.

We do not know why rocky planetary cores begin to support thick gaseous layers at about 1.5 Earth radii as opposed to 1.2 or 1.8 Earth radii, and as the community answers this question, we will learn something about planet formation,” said Lauren Weiss, a third year graduate student at UC Berkeley.

She is the second author on the group’s paper published in Proceedings of the National Academy of Sciences on May 26. The first author is Geoff Marcy the “planet hunter”, who holds the Watson and Marilyn Alberts Chair for SETI at UC Berkeley.

Not necessarily the bigger the heavier

The planets of the Solar System.
The planets of the Solar System. Image: Lsmpascal

The group analyzed the masses and radii of more than 60 exoplanets, 33 of which were discussed in the paper. “Many of the planets our study straddle the transition between rocky planets and planets with gaseous envelopes,” Weiss explained. The analysis was narrowed down to planets with orbital periods of five to 100 days, which correspond to orbital distances of 0.05 to 0.42 astronomical units. One astronomical unit (AU) is the distance between Earth and the Sun.

Fully 26.2% of such planets, which orbit Sun-like stars, have radii 1 to 1.41 times that of Earth, denoted as R, and have an orbital distance of around 0.4 AU. Accounting for planets with radii up to 4R, their prevalence jumps to more than half. In other words, one in every two planets orbiting a Sun-like star was bound to be just as wide to eight times as wide as Earth.

And in this set, the connection between exoplanet density and radius showed itself. The astronomers found that the masses of Earth-sized exoplanets steadily increased until their radii touched 1.5R, and then dropped off after. In fact, this relationship was so consistent with their data that Weiss & co. were able to tease out a relation between density and radius for 0-1.5R exoplanets – one they found held with Mercury Venus and Earth, too.

Density = 2.32 + 3.19R/R

So, the astronomers were able to calculate an Earth-like planet’s density from its radius, and vice versa, using this equation. Beyond 1.5R, however, the density dropped off as the planet accrued more hydrogen, helium and water vapor. At 1.5R, they found the maximum density to be around 7.6 g/cm3, against Earth’s 5.5 g/cm3.

The question of density plays a role in understanding where life could arise in the universe. While it could form on any planet orbiting any kind of star, we can’t also forget that Earth is the only planet on which life has been found to date. It forms an exemplary case.

There’s nothing inbetween

Are we really that alone? Photo: NASA
Are we really that alone? Photo: NASA

Figuring out how many Earth-like planets, possibly around Sun-like stars, there could be in the galaxy could therefore help us understand what the chances are like to find life outside the Solar System.

And because Earth leads the way, we think “humans would best be able to explore planets with rocky surfaces.” In the same way, Weiss added, “we would better be able to explore, or colonize, the rocky planets smaller than 1.5 Earth radii.”

This is where the astronomers hit another stumbling block. While data from Kepler showed that most exoplanets were small and in fact topped off at 4R, the Solar System doesn’t have any such planets. That is, there is no planet orbiting the Sun which is heavier than Earth but lighter than Neptune.

“It beats all of us,” Weiss said. “We don’t know why our Solar System didn’t make sub-Neptunes.”  The Kepler mission is also responsible for not providing information on this front. “At four years, it lasted less time than a single orbit of Jupiter, 11 years, and so it can’t answer questions about the frequency of Jupiter, Saturn, Uranus, or Neptune analogs,” Weiss explained.

It seems the cosmos has lived up to its millennia-old promise, then, as more discoveries trickle in on the back of yet more questions. We will have to keep looking skyward for answers.

The secrets of how planets form

Astronomers who were measuring the length of one day on an exoplanet for the first time were in for a surprise: it was shorter than any planet’s in the Solar System. Beta Pictoris b, orbiting the star Beta Pictoris, has a sunrise every eight hours. On Jupiter, there’s one once every 10 hours; on Earth, every 24 hours.

This exoplanet is located 63.4 light-years from the Solar System. It is a gas giant, a planet made mostly of gases of light elements like hydrogen and helium, and more than 10 times heavier than Earth. In fact, Beta Pictoris b is about eight times as heavy as Jupiter. It was first discovered by the Very Large Telescope and the European Southern Observatory in 2003. Six years and more observations later, it was confirmed that it was orbiting the star Beta Pictoris instead of the star just happening to be there.

On April 30, a team of scientists from The Netherlands published a paper in Nature saying Beta Pictoris b was rotating at a rate faster than any planet in the Solar System does. At the equator, its equatorial rotation velocity is 25 km/s. Jupiter’s equatorial rotation velocity is almost only half of that, 13.3 km/s.

The scientists used the Doppler effect to measure this value. “When a planet rotates, part of the planet surface is coming towards us, and a part is moving away from us. This means that due to the Doppler effect, part of the spectrum is a little bit blueshifted, and part of it a little redshifted,” said Ignas Snellen, the lead author on the Nature paper and an astronomy professor at the University of Leiden.

So a very high-precision color spectrum of the planet will reveal the blue- and redshifting as a broadening of the spectral lines: instead of seeing thin lines, the scientists will have seen something like a smear. The extent of smearing will correspond to the rate at which the planet is rotating.


Bigger is faster

So much is news. What is more interesting is what the Leiden team’s detailed analysis tells us, or doesn’t, about planet formation. For starters, check out the chart below.

Spin_rate_chart
Image: Macclesfield Astronomical Society

This chart shows us the relationship between a planet’s mass (X-axis) and its spin angular momentum (Y-axis), the momentum with which it spins on an axis. Clearly, the heavier a planet is, the faster it spins. Pluto and Charon, its moon, are the lightest of the lot and their spin rate is therefore the lowest. Jupiter, the heaviest planet in the Solar System, is the heaviest and its spin rate is also the highest. (Why are Mercury and Venus not on the line, and why have Pluto and Earth been clubbed with their moons? I’ll come to that later.)

Apparently the more massive the planet, the more angular momentum it acquires,” Prof. Snellen said. This would put Beta Pictoris b farther along the line, possibly slightly beyond the boundaries of this chart – as this screenshot from the Leiden team’s pre-print paper shows.

planet_spin_rate1

Unfortunately, science doesn’t yet know why heavier planets spin faster, although there are some possible explanations. A planet forms from grains of dust floating around a star into a large, discernible mass (with many steps in between). This mass is rotating in order to conserve angular momentum. As it accrues more matter over time, it has to conserve the kinetic and potential energy of that matter as well, so its angular momentum increases.

There have been a few exceptions to this definition. Mercury and Venus, the planets closest to the Sun, will have been affected by the star’s gravitational pull and experienced a kind of dragging force on their rotation. This is why their spin-mass correlations don’t sit on the line plotted in the chart above.

However, this hypothesis hasn’t been verified yet. There is no fixed formula which, when plotted, would result in that line. This is why the plots shown above are considered empirical – experimental in nature. As astronomers measure the spin rates of more planets, heavy and light, they will be able to add more points on either side of the line and see how its shape changes.

At the same time, Beta Pictoris b is a young planet – about 20 million years old. Prof. Snellen used this bit of information to explain why it doesn’t sit so precisely on the line:

planet_spin_rate2

Sitting precisely on the line would be an equatorial velocity of around 50 km/s. But because of its youth, Prof. Snellen explained, this exoplanet is still giving off a lot of heat (“this is why we can observe it”) and cooling down. In the next hundreds of millions of years, it will become the size of Jupiter. If it conserves its angular momentum during this process, it will go about its life pirouetting at 50 km/s. This would mean a sunrise every 3 hours.

I think we can stop complaining about our days being too long.


Spin velocity v. Escape velocity

Should the empirical relationship hold true, it will mean that the heaviest planets – or the lightest stars – will be spinning at incredible rates. In fact, the correlation isn’t even linear: even the line in the first chart is straight, the axes are both logarithmic. It is a log-log plot where, like shown in the chart below, even though the lines are straight, equal lengths of the axis demarcate exponentially increasing values.

log-log
Image: Wikipedia

If the axes were not logarithmic, the line f(x) = x3 (red line) between 0.1 and 1 would look like this:

plot3
Image: Fooplot.com

The equation of a line in a log-log plot is called a monomial, and goes like this: y = axk. In other words, y varies non-linearly with x, i.e. a planet’s spin-rate varies non-linearly with its mass. Say, if k = 5 and a (a scaling constant) = 1, then if x increases from 2 to 4, y will increase from 32 to 1,024!

Of course, a common, and often joked-about, assumption among physicists has been made: that the planet is a spherical object. In reality, the planet may not be perfectly spherical (have you known a perfectly spherical ball of gas?), but that’s okay. What’s important is that the monomial equation can be applied to a rotating planet.

Would this mean there might be planets out there rotating at hundreds of kilometres per second? Yes, if all that we’ve discussed until now holds.

… but no, if you discuss some more. Watch this video, then read the two points below it.

  1. The motorcyclists are driving their bikes around an apparent centre. What keeps them from falling down to the bottom of the sphere is the centrifugal force, a rotating force that, the faster they go, pushes them harder against the sphere’s surface. In general, any rotating body experiences this force: something in the body’s inside will be fleeing its centre of rotation and toward the surface. And such a rotating body can be a planet, too.
  2. Any planet – big or small – exerts some gravitational pull. If you jumped on Earth’s surface, you don’t shoot off into orbit. You return to land because Earth’s gravitational pull doesn’t let you go that easy. To escape once and for all, like rockets sometimes do, you need to jump up on the surface at a speed equal to the planet’s escape velocity. On Earth, that speed is 11.2 km/s. Anything moving up from Earth’s surface at this speed is destined for orbit.

Points 1 and 2 together, you realize that if a planet’s equatorial velocity is greater than its escape velocity, it’s going to break apart. This inequality puts a ceiling on how fast a planet can spin. But then, does it also place a ceiling on how big a planet can be? Prof. Snellen to the rescue:

Yes, and this is probably bringing us to the origin of this spin-mass relation. Planets cannot spin much faster than this relation predicts, otherwise they would spin faster than the escape velocity, and that would indeed break the planet apart. Apparently a proto-planet accretes near the maximum amount of gas such that it obtains a near-maximum spin-rate. If it accretes more, the growth in mass becomes very inefficient.

(Emphasis mine.)


Acting forces

The answer will also depend on the forces acting on the planet’s interior. To illustrate, consider the neutron star. These are the collapsed cores of stars that were once massive but are now dead. They are almost completely composed of neutrons (yes, the subatomic particles), are usually 10 km wide, and weigh 1.5-4 times the mass of our Sun. That implies an extremely high density – 1,000 litres of water will weigh 1 million trillion kg, while on Earth it weighs 1,000 kg.

Neutron stars spin extremely fast, more than 600 times per second. If we assume the diameter is 10 km, the circumference would be 10π = ~31 km. To get the equatorial velocity,

Vspin = circumference × frequency = 31 × 600/1 km/s = 18,600 km/s.

Is its escape velocity higher? Let’s find out.

Ve = (2GM/r)0.5

G = 6.67×10-11 m3 kg-1 s-2

M = density × volume = 1018 × (4/3 × π × 125) = 5.2×1020 kg

r = 5 km

∴ Ve = (2 × 6.67×10-11 × 5.2×1020/5)0.5 =  ~37,400 km/s

So, if you wanted to launch a rocket from the surface of a neutron star and wanted it to escape the body’s gravitational pull, it has to take off at more than 30 times the speed of sound. However, you wouldn’t get this far. Water’s density should have given it away: any object would be crushed and ground up under the influence of the neutron star’s phenomenal gravity. Moreover, at the surface of a neutron star, the strong nuclear force is also at play, the force that keeps neutrons from disintegrating into smaller particles. This force is 1032 times stronger than gravity, and the equation for escape velocity does not account for it.

However, neutron stars are a unique class of objects – somewhere between a white dwarf and a black hole. Even their formation has nothing in common with a planet’s. On a ‘conventional’ planet, the dominant force will be the gravitational force. As a result, there could be a limit on how big planets can get before we’re talking about some other kinds of bodies.

This is actually the case in the screenshot from the Leiden team’s pre-print paper, which I’ll paste here once again.

planet_spin_rate1

See those circles toward the top-right corner? They represent brown dwarfs, which are gas giants that weigh 13-75 times as much as Jupiter. They are considered too light to sustain the fusion of hydrogen into helium, casting them into a limbo between stars and planets. As Prof. Snellen calls them, they are “failed stars”. In the chart, they occupy a smattering of space beyond Beta Pictoris b. Because of their size, the connection between them and other planets will be interesting, since they may have formed in a different way.

Disruption during formation is actually why Pluto-Charon and Earth-Moon were clubbed in the first chart as well. Some theories of the Moon’s formation suggest that a large body crashed into Earth while it was forming, knocking off chunks of rock that condensed into our satellite. For Pluto and Charon, the Kuiper Belt might’ve been involved. So these influences would have altered the planets’ spin dynamics, but for as long as we don’t know how these moons formed, we can’t be sure how or how much.

The answers to all these questions, then, is to keep extending the line. At the moment, the only planets for which the spin-rate can be measured are very massive gas giants. If this mass-spin relation is really universal, than one would expect them all to have high spin-rates. “That is something to investigate now, to see whether Beta Pictoris b is the first of a general trend or whether it is an outlier.”


Reference:

Fast spin of the young extrasolar planet β Pictoris b. Nature. doi:10.1038/nature13253