Newly Discovered Super-Earths Present A Golden Opportunity To Study Exoplanet Atmospheres

An artist's impression of the newly discovered members of the Gliese 887 system and their star, but the most exciting possibility of all is not pictured. Mark Garlick

The quest to find planets around stars near enough to learn their composition has received a boost with the discovery of two rocky worlds around Gliese 887. Although the inner of the two new discoveries is likely far too hot for life, the other might just be able to support liquid water. What is really exciting astronomers, however, is the combination of the relative closeness of the system, the hint of a third planet, and parental star characteristics that make these worlds particularly well suited for deeper exploration.

Most stars are M-type (red dwarves). However, they're not what we see staring at the night sky. These stars are so faint that even the closest star to the Sun, Proxima Centauri, requires a substantial telescope to glimpse it. Gliese 887 (also known as Lacaille 9352) is the brightest red dwarf, being relatively big by M-type standards (around half the Sun's mass), and comparatively close. At 11 light-years away, it's the 12th closest star system. It's still not bright enough to see with the naked eye unless your vision is superb and the conditions are ideal.

In Science, the RedDots team have announced the discovery of two planets in the system, and the suspected presence of a third, all discovered using the radial velocity (Doppler wobble) method. Gliese 887b has an orbit of just 9.3 days, while Gliese 887c takes 22 days and is estimated to have an average temperature of 79ºC (174ºF). Their masses are approximately 4.2 and 7.6 times that of the Earth, respectively. If 887d exists, it is thought to have a 51-day year, which would make it just slightly warmer than Earth.

The combination of closeness and moderate size also makes Gliese 887 exactly the sort of star astronomers want to find hosting planets. The advantages of being close are obvious for further research, while the star's mass and brightness hit something of a sweet spot. For a bright star like the Sun, any habitable planets have to be located far enough away that their orbits take frustratingly long to complete. Fainter ones tend to be prone to energetic outbursts that would likely sterilize any planet close enough to be habitable.

These outbursts also make planets around such stars hard to study. The Gliese system is, in the words of Professor Melvyn Davies of Lund University in an accompanying editorial, “multiple, quiet and close by.” Its starspots are much less abundant than the Sun's, let alone most red dwarves.

Future generations of telescopes, such as the James Webb, have a good chance of viewing the planets' atmospheres and measuring their composition unimpeded. For the known planets, that could be a big step forward for our exoplanet research. If Gliese 887d exists, it'd be among our best chances of finding life beyond the Solar System.

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