New observations from the Hubble Space Telescope are helping characterize the atmospheres of exotic exoplanets.
Theory says that the super-Earth 55 Cancri e contains crystallized carbon in its interior, earning it the nickname “diamond planet.” Theory also says that on the hot, young super-Jupiter 2M1207, rain could be made of vaporized rocks, silicates as fine as cigarette smoke particles. Deeper within its atmosphere, that rain may turn to iron sleet.
Astronomers base these ideas on scant information, garnered when an exoplanet passes in front of its host star or tugs the star back and forth: mass, radius, and average density if they’re lucky. But two recent studies use the Hubble Space Telescope’s infrared camera to put these exotic theories on firmer observational ground.
The Diamond Planet
Of course, 55 Cancri e isn’t really made of diamonds, nor could we ever mine it even if it were. This extraordinarily dense exoplanet eight times Earth’s mass and twice its radius circles a star 40 light-years away. Yet the planet is too big to have a silicate interior like Earth’s, unless it’s also covered by a gaseous envelope or an ocean. Both of these scenarios would have trouble surviving on a planet whose close-in orbit (with a period less than a day) brings its surface to a boiling 2000K (3100°F). In particular, any water would have to be in a supercritical state to remain a liquid.
That’s why some theorists have taken a less Earth-centric approach and suggested that carbon (and the dense structures it can create) might dominate the exoplanet’s chemistry rather than oxygen-rich silicates.
New observations are bearing out that idea. Angelos Tsiaras (University of College London) and his team used Hubble’s Wide Field Camera 3 to split the star's light into a spectrum - after the light has passed through the planet's atmosphere. Even thought it’s rough and noisy, this transmitted spectrum makes clear that 55 Cancri e’s air must be mostly made of hydrogen and helium.
The team also found tantalizing hints of hydrogen cyanide (HCN, also known as prussic acid), a molecule that would only dominate in a carbon-rich environment. But confirmation awaits future telescopes such as the James Webb Space Telescope. And the team found no trace of water vapor, which would form easily if oxygen were widespread on this planet.
This marks the first successful measurement of a super-Earth’s atmospheric composition. According to Kepler data super-Earths are one of the most common types of planet in our galaxy, yet they have no representative in our solar system. Thick cloud cover probably stymied previous attempts to measure atmospheres around two other super-Earths, GJ 1214b and HD 97658b.
A Hot, Young Super-Jupiter
Not so very far from 55 Cancri e is the super-Jupiter known as 2M1207b — by celestial coincidence, it lies 170 light-years from both 55 Cancri e and Earth. The gas giant circles its brown dwarf host at a distance of roughly 40 astronomical units, equivalent to Pluto’s average traverse from the Sun.
Though it carries roughly four times Jupiter’s mass, this planet is only a fraction of Jupiter’s age at a tender 10 million years old, so it’s still contracting and radiating heat. When Hubble first imaged this planet 10 years ago, it found cloud-top temperatures that soared to more than 1500K (2200°F). These are the conditions that led theorists to suggest that rock and iron rain might form in this planet’s atmosphere. (The exotic weather will fade as the hotheaded adolescent ages and cools over the next billion years.)
While the ability to observe this planet’s weather patterns is still a little ways off, researchers have taken the first steps to better understanding its atmosphere. Yifan Zhou (University of Arizona) and his team turned once more to Hubble to watch the planet’s face rotate for more than 8 hours in a technique known as rotational mapping.
Although the observations didn’t last long enough to cover the full orbital period, they were enough to settle on a spin rate of about 10 to 12 hours, roughly the same as Jupiter. Future observations may probe deeper, as they’ve done for more massive brown dwarfs, to reveal cloud structure and evolution.