Last year, astronomers discovered a remarkable planet orbiting another star: it has a mass and radius that puts it in the "super-Earth" category -- meaning it's more like the Earth than a giant Jupiter-like planet. Today, it has been announced that astronomers have been able to analyze the atmosphere of the planet (the very first time this has ever been accomplished for a super-Earth), and what they found is astonishing: the air of the planet is either shrouded in thick haze, or it's loaded with water vapor... in other words, steam!
[Click to embiggen the artist illustrations of the planet and star.]
This is very cool news. Um, hot. Whatever.
Here's the deal: GJ 1214 is a dinky red dwarf star 42 light years away. It's only about 1/5th the size of the Sun, and shines with only 1/300th of the Sun's brightness. A project called MEarth studies such nearby red dwarfs, looking for dips in their starlight that indicate the presence of a planet: when the planet passes in front of the star (called a transit), it blocks the light a little bit. The cool thing about transits is that if we know the radius of the star and how much the light dips, we can immediately get the size of the planet! The bigger the planet, the more light is blocked. If the planet blocks, say, 1% of the light, then it has a radius 1/10th that of the star (the area of the disk of the planet is related to the radius of the planet squared, so if the planet's radius is 0.1 times the star's, then the area of the planet's disk compared to the star's disk is 0.1 * 0.1 = 0.01 = 1%).
In 2009, astronomers found just such a dip in GJ 1214's light, meaning there was a planet there (called GJ 1214b). The radius of the planet turns out to be about 2.6 times that of the Earth. That's much bigger than we are, but still much smaller than Jupiter (which is 11 times the Earth's diameter). But don't go thinking it's Earthlike: it orbits the red dwarf at a distance of only 2 million kilometers, screaming around the star once every 38 hours! Even though the star is much cooler than our Sun, from that distance the planet gets cooked to a temperature of about 200° Celsius (~400° F). Ouch.
The thing is, the way the light dipped indicated the planet was bigger than models indicated it should be. One thing that can do that is an atmosphere, in this case one about 200 km (120 miles) thick -- much thicker than ours.
OK, got that? Astronomers have detected that a planet 420 trillion kilometers away -- 250 trillion miles -- has an atmosphere!
That in and of itself is incredible. But wait! There's more.
Astronomers observed the planet when it passed in front of the star, analyzing the light very carefully. As starlight passes through the planet's atmosphere, certain colors of it get absorbed, and these are like fingerprints that can be used to figure out the atmospheric composition. Most models predicted a heavy hydrogen content, but the observations indicate none is there! That means either there are thick layers of haze in the upper atmosphere of the planet, obscuring any hydrogen below them -- much like Venus or Saturn's moon Titan, blocking the view lower down -- or there is a vast amount of water in the planet's air. And at a temperature of 200° C, that water would be in the form of vapor. In other words, steam.
It's unclear which scenario is more likely, but either way this is an amazing accomplishment. I suspect (opinion time here!) that water vapor is the culprit; according to the astronomers' data, there aren't any known haze particles at the pressure and temperature indicated that could form clouds thick enough to explain the observations. That doesn't mean there is no haze; just that current models of how these processes work come up empty for this situation. And more observations may yet be able to distinguish between haze and water vapor in the planet's air.
But there's something else I was to add, something I haven't seen in the papers or the press releases. Observations of how hard the planet is tugging on its star have yielded the mass of the planet: it's 6.5 times as beefy as we are. So we have the mass and the radius of the planet, and that means we can calculate many other features, like its density -- which turns out to be very low, only about 1/3 the density of the Earth! That means this planet must be very deficient in heavy metals compared to the Earth, or else it would be much denser.
Interestingly to me, having the mass and radius also means we can find the surface gravity of the planet: in other words, how strongly gravity would pull you down if you stood on its surface. Given the large mass, you might expect the gravity to be much stronger than Earth's, but in fact when I did the math I was surprised to find that the surface gravity is almost exactly the same as we feel here on Earth!
I know this is a distant world, much larger, more massive, and hotter than Earth, shrouded in a thick atmosphere far different than ours, orbiting a Sun about as different from ours as can be... but weirdly, knowing I'd weigh about the same standing there as I do here somehow makes the planet seem a lot less, well, alien.
It's easy to forget that these aren't just distant points of light, or simple artist drawings. These are worlds. And every one of them is different, strange, wonderful, and awe-inspiring.
And this new result serves as a brilliant reminder: we live in an age where we can taste the air of alien planets from trillions of kilometers away!
I can think of no better paean to science.
Artist illustrations credits: ESO/L. Calçada