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High on the wish-list of astronomers studying planets outside the Solar System (exoplanets) is an Earth-like world with the right combination of circumstances to study its atmosphere. Now they've finally found one, although it probably resembles Venus more than Earth.
With rare exceptions, the exoplanets we have discovered were found either by observing wobbles in the star's movement from the planet's gravity or dips in a star's brightness when the planet passes in front. The first method reveals the planet's mass, the second its size, and possibly also the hope of starlight passing through an atmosphere. Both are needed to understand a planet, so the rare planets that can be detected both ways are precious.
Gliese 486b is one such example and, a paper in Science reveals it has several other key features that make it suitable for atmospheric searches. Although Gliese 486b is not suited to supporting life itself, the things we could learn from it could be invaluable for narrowing down the search for habitable planets elsewhere.
Atmospheres around planets are important because they may be a sign that life could potentially survive there. Earth's atmosphere sustains life by trapping heat and warming the planet but also protects it from the Sun's harmful radiation. Mars on the other hand has a very thin atmosphere, meaning the planet is colder and less protected, hence appearing as an inhospitable landscape.
Only planets with orbits aligned with Earth can be detected through dips in their star's brightness. Planets need large masses compared to their star to gravitationally induce wobbles we can see. It also helps if the star is nearby, and not moving around too much for other reasons. Gliese 486b combines all of these.
The star Gliese 486 is 26 light-years away — practically in our galactic neighborhood — and is a red dwarf with a mass of 32 percent of the Sun's. That makes it light enough to be noticeably thrown around by Gliese 486b, a “super-Earth” with mass 2.8 times our own planet.
It took four years of ground-based monitoring of Gliese 486's spectrum to learn it is orbited by a planet every 1.47 days. NASA's planet-hunting TESS space telescope needed to see the same world pass just 13 times across its face to convince the paper's authors the dips in brightness they saw came from the same world, not starspots. This process also revealed the star is quiet enough to allow us to observe the planet's atmosphere without interference.
Gliese 486b is so close to its star the temperature is estimated at 700 K (800ºF), only slightly cooler than Venus, putting life out of the question. However, there is a big debate as to whether rocky planets orbiting close to red dwarfs can hold onto their atmospheres at all in the face of the stars' frequent flares. Detecting an atmosphere would therefore tell us something important, let alone learning its composition.
We do know a little about the atmospheres of some exoplanets, including seriously hardcore places where its likely to rain iron. However, these are gas giants like Jupiter, rather than places with even a passing resemblance to our home. Rocky planets are, by definition, smaller than gas giants, making their atmospheres harder to study, so everything has to be just right to make it happen.
