Using NASA's Hubble Space Telescope, an international team of scientists has tracked two planets crossing in front of the same star at the same time -- discovering that these worlds are both rocky and have comparatively thin atmospheres.
The findings, described in the journal Nature, lend growing support to the idea that such planets might potentially be friendly to life.
This spring, a team of astronomers using the TRAnsiting Planets and PlanetesImals Small Telescope (or TRAPPIST for short) in the Atacama Desert in Chile announced that they had discovered TRAPPIST-1, a nearby star just 40 light-years from Earth that hosts three potentially habitable, approximately Earth-sized exoplanets.
Though the innermost planets are so close to the star that they take just 1.5 and 2.4 Earth days to circle it, they probably have pretty moderate temperatures, perhaps in the range to allow liquid water on the surface. That's because TRAPPIST-1 is an ultra-cool dwarf star, just a little larger than Jupiter and exceedingly dim. Were you to stand on one of TRAPPIST-1's planets, the star would appear to have a low, dark-red glow.
Of course, whether there's water (and the potential for life) on any of these planets depends on what kind of atmospheres they might have — something scientists had not yet found out.
Then, shortly before announcing the triple-find, the researchers realized that two of the planets would be transiting the star nearly simultaneously, allowing the scientists a rare opportunity to study the planets' atmospheres in one go.
"We could actually do this for two planets at the same time ... killing two birds with one stone," said lead author Julien de Wit, an astronomer at MIT.
There was one hitch: The event was happening very soon — two weeks from their realization.
The scientists had to scramble to put together a proposal for time using the Hubble Telescope, sending it off within 24 hours. The proposal was quickly reviewed and, to their relief, approved.
"That was really, really, really last minute," De Wit said, "but fortunately it worked out."
To study the planets' atmospheres, the scientists used a technique called transmission spectroscopy.
As the planets (TRAPPIST-1b and TRAPPIST-1c) cross in front of their host star, the starlight streams through the thin skin of gas and dust that may make up the two worlds' atmospheres. The atoms and molecules clinging to the planet's surface will absorb certain wavelengths of light, while letting others pass through unscathed. In other words, the planets' atmospheres looks opaque in some wavelengths but transparent in others.
If you were theoretically able to see either or the planets silhouetted against its star in the transparent wavelengths, the dark circle would only look as big as the actual planet's surface, because the light passing through the atmosphere would still reach Earth. But seen in the wavelengths that are absorbed by that planet's atmosphere, that dark circle would look bigger, because the atmosphere would also be blocking those wavelengths of light.
Essentially, by subtracting one measurement from the other, you can tell how thick the actual atmosphere is.
That's sort of the general idea, anyway. In fact, the scientists are looking at dips in starlight, to see how deep they are in the different wavelengths and how much they vary across those wavelengths.
If the dips in starlight had been deep and varied over a wide range of wavelengths, then the planets would likely have thick, puffy, hydrogen-rich envelopes like our solar system's gas giants. However, Hubble showed that the dips occurred only over a narrow range of wavelengths and didn't vary much, which means that they probably have atmospheres akin to those of Earth, Mars or Venus.
Of course, Earth, Mars and Venus have (from a human perspective) extremely different atmospheres, even though they're all rocky planets of about the same size in about the same area. Venus' air is thick with carbon dioxide and broils the planet's surface (so much that its surface is technically hotter than that of Mercury, the planet closest to the sun). Mars, on the other hand, has lost the vast majority of its atmosphere and has only the thinnest layer still clinging to the surface. Earth, which sits between the two, has the just-right atmosphere: thick enough to keep liquid water stable but thin enough to keep from stifling us.
What kind do these two planets have?
The devil remains in the chemical details. De Wit says that Hubble should be able to pick out some characteristics, such as whether their atmospheres contain water or methane, but that the plaenets will probably best be picked apart by future missions such as NASA's James Webb Space Telescope, set to launch in 2018, which will be able to peer deeper into the infrared light spectrum than Hubble can.
"With Hubble we can lift the veil a bit more, but with James Webb we'll be able to pin that down," he said, "to really reveal what are the different components, the temperature, the pressure — so the whole story."
The scientists are hoping to raise funding very soon to build similar telescopes that will be able to find more ultra-cool dwarf stars with similar planets, because these worlds would serve as ideal targets for James Webb to study once it comes online.