Accurate measurement of exoplanet radius
Image: Imaginative illustration of Kepler 93b’s diameter being measured. Credit: NASA/JPL-Caltech
Using both the Kepler and the Spitzer space telescopes, scientists from NASA have made the most precise measurement of an exoplanet’s radius yet. Kepler 93b, which orbits a dim star 300 ly away, has a diameter of 18,800 km, give or take 240 km. “The measurement is so precise that it’s literally like being able to measure the height of a six-foot tall person to within three-quarters of an inch – if that person were standing on Jupiter,” said Sarah Ballard, an astronomer at the University of Washington and lead author of a paper in The Astrophysical Journal that describes the findings, in a JPL statement.
Kepler 93b is a super-earth, a common class of planets in the Milky Way but missing in the Solar System. Super-earths weigh between the masses of Earth and Uranus. Scientists were able to its radius to within 240 km by first using the Kepler space telescope to record how much of starlight the exoplanet blocked when transiting across its face. Next, they used precise measurements of seismic waves moving within the star’s interior to calculate how much light it gave off and its radius. This technique falls within the field of astroseismology that has been used since the early 2000s. Astroseismic measurements are effective when the observatories have a long baseline, long observing time and high photometric precision.
The scientists were aided in their work by Kepler 93 being a cool dwarf star whose brightness varies less often and strongly enough to help constrain planetary transit and seismic measurements.
Then, the Spitzer space telescope used its Infrared Array Camera, or IRAC, to confirm that what Kepler was observing wasn’t a false-positive. It did this by using the fact that no matter which wavelength a transiting exoplanet is observed in, its transit depth will be the same. The transit depth is the ratio of the size of a planet’s disk to the star’s disk. So while Keplre measures this ratio in visible light, the IRAC will measure it in infrared light. To rule out a false-positive, the two measurements have to be the same.
The IRAC measurement was improved using a method developed in 2011, which checks how light falls on individual pixels in the camera. The scientists used Kepler 93b as a test bed, examining the exoplanet’s seven transits recorded between 2010 and 2011 in detail. Based on its mass – 3.8-times Earth’s – and radius, it was found to be made mostly of iron and rock, its biggest similarity to Earth. However, it orbits its star at a distance almost 19 times shorter than that between us and our Sun, making its surface too hot for life at 760 degrees Celsius.
Spitzer lost its coolant, and therefore the sensitivity of some of its instruments, in 2009. A telescope that measures heat coming in from various regions of the cosmos must have little heat of its own, which the cryogen ensured. Once it ran out, the temperature inside the telescope rose by 29 degrees Celsius, too warm for longer wavelength instruments but still cold enough for shorter wavelength ones like IRAC. The precision of the Kepler 93b measurement will give hope for future studies to understand why and how super-earths form, and instruments like IRAC will play an important role in that scenario.
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References
Sarah Ballard et al., Kepler 93b: A terrestrial world measured to within 120 km, and a test case for a new Spitzer observing mode, 2014 ApJ 790 12 doi:10.1088/0004-637X/790/1/12 (pre-print)
JPL press release: The Most Precise Measurement of an Alien World’s Size, July 23, 2014
