Even after the loss of a critical stabilization system on-board, the NASA Kepler space telescope has made an important discovery. Scientists from the University of California have used the telescope to find a nearby star-system comprising a cool red M-dwarf and three planets slightly larger than Earth orbiting it. The find’s significance stems from various reasons, not the least of which is that this system is only 150 light-years away, close enough for astronomers to make direct observations of the star and the planets’ atmospheres*.
The three planets’ radii are 2.1-, 1.7- and 1.5-times that of Earth. The outermost planet, with the radius 1.5 RE, is in fact on the inner edge of the system’s habitability zone and receives 1.4-times the light that Earth does from our Sun. Moreover, follow-up observations made from the Automated Planet Finder telescope, California, and the Keck Telescope, Hawaii, indicate that the planets’ surfaces are cool and not scorched as exoplanets’ surfaces have often been found to be. The ‘lukewarm’ temperature is a sign that the planets are fully-formed.
For these reasons and others, finding this star-system has been like striking gold for astronomers. Its proximity permits them to closely monitor its evolution than if it had been farther away. The red M-dwarf at its center – designated EPIC 201367065 – is not too bright or its electromagnetic flux would have ‘bleached’ out observations; its moderate emission also means the planets’ surfaces aren’t scorched. The almost Earth-sized planet just about in the habitable zone means they can study if its surface conditions are conducive to life (A recent analysis concluded that one in five Sun-like stars in the Milky Way hosts an Earth-sized planet in the habitable zone, which means there are 40 billion such planets in our galaxy alone). Making matters easier overall is the proximity of the system itself, which also means investigations can be more detailed for the same resources.
One such detail that has not been explored with any great precision among farther exoplanets is composition. With sizes in the 1.5-2.1 RE range, the study’s authors think “they may span the gap between rock-dominated ‘Earths’/’super-Earths’ and low-density ‘sub-Neptunes’ with considerable volatile content”. Compositional analyses are important to understand what kind of planets can form under what conditions and how their orbits could have migrated within the system before attaining equilibrium. Additionally, they could also help astronomers understand why there are no planets heavier than Earth but lighter than Neptune in our Solar System.
Anyway, the next course of action will be to use the Hubble space telescope to compose a spectroscopic map of the outermost planet’s atmosphere. Many exoplanets that possess atmospheres also possess hydrogen-rich atmospheres, with no hints of the oxygen and nitrogen that have been able to support life on Earth. If the outermost planet’s atmosphere is also dominated by hydrogen, then the gas’s presence will show up in Hubble’s measurements. As the study’s lead author Ian Crossfield, from the Lunar & Planetary Laboratory at the University of Arizona, noted, the presence of large quantities of hydrogen doesn’t preclude life but only life as we’ve known it on Earth.
The study’s paper was uploaded to the arXiv pre-print server on January 15 and has been submitted to the Astrophysical Journal.