Artist Rendition of exoplanet GJ 436b and it's tail. Image credit: Mark Garlick/University of Warwick

Jul 2, 2015 Planet Tails

Sen—When you think of a planet, you don't tend to think of them as having tails like comets, but that's exactly what exoplanet GJ 436b has. Researchers have recently announced that the giant planet is surrounded by a co-moving and trailing cloud of hydrogen gas. The atmosphere of GJ 436b is thought to be composed mostly of hydrogen (like our own Jupiter and Saturn) and, due to interactions with its star's radiation in X-ray and ultraviolet, the planet's atmosphere is literally being blown away.

GJ 436b is a Neptune-mass gas giant planet whipping around its parent star every 2.64 days. Originally discovered through the radial velocity technique in 2004, it is the luck of nature that GJ 436b also transits its parent star, as viewed from the Earth, that enabled this discovery. Astronomer David Ehrenreich and collaborators set out to use NASA's Hubble Space Telescope and Chandra X-ray Observatory in a coordinated effort to study GJ 436b's atmosphere and the activity of its parent star. They found hydrogen gas beyond the planet’s atmosphere surrounding and flowing away from the world. This tail of hydrogen is huge—about 50 times larger in size than the red dwarf star the planet orbits. 

This cloud of gas being spewed into space was detected by observing the planet as it passed in front of its star whilst watching the star in ultraviolet (UV). In optical light the gas cloud is too transparent to be seen, but the hydrogen atoms absorb the UV photons, making the extended hydrogen cloud trailing GJ 436b 'visible' in transits observed at those wavelengths. These observations show that compared to the optical transit, when you stare in the UV, you find a longer and asymmetric transit. Using spectroscopy, the researchers were able to detect the presence of the hydrogen cloud and measure the speed it is flowing.

We often discuss and talk about exoplanets as isolated entities, but this discovery is a poster child for how important interactions between a planet and its host star can be, and a good reminder that planets are being found in large and dynamic solar systems. The radiation from the parent star is basically evaporating the atmosphere, sufficiently energizing hydrogen atoms that they can escape gravitationally unbound from the atmosphere. The cloud forms a comet-like tail as a result of the starlight's radiation pressure pushing on the escaped hydrogen. Since this star is cooler and less luminous than our Sun, the gas cloud is not completely swept away, but enshrouds GJ 436b and forms a tail.

After its formation the Earth is thought to have held on for a time to a hydrogen atmosphere that is now long gone. It was probably lost due to interactions with the young Sun's ultraviolet and X-ray radiation in a similar mechanism to what is currently happening at GJ 436b.

GJ 436b is not in danger of losing its atmosphere any time soon. The calculated rate of gas escape at the present time is too low to deplete the atmosphere over the entire age of its parent star's life. GJ 436b will remain a gas giant, but the fates of other planets closer to more active stars or smaller in size might not be as lucky. This process could strip all of the atmosphere leaving only the rocky core of a former giant planet.

This has implications for planet formation and the origin of the collection of rocky and super-Earth-sized planets discovered in orbit close about their parent stars. It's possible that instead of forming as terrestrial planets in place, rocky planets orbiting their stars every few days formed further out beyond the snow line where they accreted large amounts of gas before migrating and being stripped of their atmospheres. This idea has been suggested before and there has been some circumstantial evidence for atmospheric escape, but this discovery lends the first evidence in direct support.

GJ 436b with its gas cloud is the prototype of a new class of planets. You might ask why we can't just search all the transiting hot Jupiters and hot Neptunes found by the Kepler and CoROT spacecrafts. Unfortunately most of those planets and planet candidates are around stars that are too faint for these observations, so we need a plethora of close-in planets around bright stars.

The prospects of finding these types of planetary systems are very good. The current and next-generation space-based transit surveys, K2 and the Transiting Exoplanet Survey Satellite (TESS), are focused on finding large planets on short orbits (less than 75 days) around the brightest stars in the sky. K2 using Kepler's, ~100 square degree field of view is now active, monitoring a different set of ~10,000 stars ever 75 days. TESS is expected to launch in 2017 with its primary mission to monitor the 500,000 brightest and nearby stars for the signs of planets on orbits less than 30 days. The stars that TESS will search will be 30-100 times brighter than ones the original Kepler mission monitored. With a larger sample, planets at varying stages of atmospheric loss will be found that confirm whether or not the majority of close in rocky planets are the burnt embers leftover of gas giants who ventured to close to their host stars.

Artistic animation representing what the transit of GJ 436b and its hydrogen tail would look like if you could see in the ultraviolet Video Credit: NASA, ESA, STScI, Martin Kornmesser (ESA/Hubble)

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