The field of exoplanetary research has revealed an unexpected plethora of very different and strange types of planets around other stars during the last 20 years, from scorching hot, massive hot-Jupiters to super-Neptune-type, “puffy” planets to solid terrestrial ones several times bigger than our home planet, which are mostly made of diamond and graphite. Now, astronomers are adding one more strange type of alien worlds into the mix: that of comet-like, helium-rich, hot-Neptunes which experience a severe loss of their atmospheric hydrogen into space.
This new strange member of the exoplanetary zoo is called Gliese 436b, or GJ 436b, which had been discovered with the transit method in 2004, in orbit around a nearby dim red dwarf star approximately 33 light-years away in the direction of the constellation Leo. Subsequent observations had determined Gliese 436b to be a Neptune-sized world, just slightly larger and more massive than the ice giants Uranus and Neptune in our own Solar System. What was significant about this strange alien world was its orbital period of 2.6 days, which meant that the planet was no more than 4 million km away from its host star, making it one of the first “warm Neptunes” ever to be discovered, in a similar fashion to the much larger and more massive “hot-Jupiters” that had also been found by the hundreds around other stars.
Since GJ 436b was thought to have an atmospheric chemical composition similar to Neptune, which is rich in hydrogen and helium and methane, astronomers expected to see a similar structure in the atmosphere of GJ 436b as well. Yet, contrary to theoretical predictions, a series of observations of GJ 436b with NASA’s Hubble and Spitzer space telescopes in recent years revealed a surprising absence of atmospheric methane and at the same time a great abundance of carbon monoxide. Several models have tried to explain this discrepancy, by postulating that the atmosphere of GJ 436b may indeed be harboring a hydrogen-rich atmosphere but with an unusually high carbon-to-oxygen ratio, which could help account for the abundance of carbon monoxide and the subsequent absence of methane, as has been observed by Hubble and Spitzer. Such a chemistry, however, would require an unusually high metallicity (the amount of elements heavier than hydrogen and helium) in the planet’s atmosphere that would result in a planet with a much greater temperature and mass than the ones observed.
Now, a team of astronomers from the U.S. have proposed an alternate explanation for the observed atmospheric properties of GJ 436b, in a recent study published at The Astrophysical Journal. In their study, the researchers propose that what has been observed in the atmosphere of GJ 436b is the result of a gradual loss of hydrogen as a result of ultraviolet irradiation from the planet’s host star, leaving behind an atmosphere that is dominated by helium. Such a process isn’t observed in any of the planets in our Solar System, since all of them (and especially the ice giants Uranus and Neptune) orbit the Sun at such great distances that solar irradiation pressure has a miniscule effect in their atmospheric dynamics. But in the case of GJ 436b, which orbits its host star almost 15 times closer than Mercury orbits the Sun, stellar radiation pressure becomes a significant factor. Since its host star has an age of approximately 6 billion years, GJ 436b would have more than enough time to transition from an initial hydrogen-helium atmosphere to a helium-dominated one as a result of a constant escape of hydrogen.
“We therefore propose that exoplanet GJ 436b may have a helium-dominated atmosphere that evolved from a primordial hydrogen and helium envelope,” write the researchers in their study. “The proximity of the planet to its parent star provides the conditions to maintain transonic hydrodynamic escape throughout the evolution history. The mass and the size of the planet determine whether the escape rate has been close to the diffusion-limited escape rate of hydrogen. As a result, the planet has experienced disproportional loss of its primordial hydrogen. Some of the primordial helium has also been lost during this evolution, but some may have remained … We find that a helium-dominated atmosphere with a low hydrogen abundance and close-to-solar carbon and oxygen abundances can adequately fit both [Hubble and Spitzer] datasets of GJ 436b’s emission spectrum.”
As the researchers point out, the exact amount of hydrogen loss depends on the mass of the planet’s initial atmosphere, but the results of their study showed that GJ 436b’s hydrogen abundance could decrease by more than an order of magnitude on a time scale of a few billion years, while the overall depletion of its atmospheric hydrogen could take up to 10 billion years. “Hydrogen is four times lighter than helium, so it would slowly disappear from the planets’ atmospheres, causing them to become more concentrated with helium over time,” says Renyu Hu, a Hubble fellow at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and lead author of the study. “The process would be gradual, taking up to 10 billion years to complete.” For reference, our planet Earth is about 4.5 billion years old.
A second independent study, which was based on additional observations of GJ 436b by the Hubble Space Telescope, comes to confirm the results by Hu’s team, while revealing the presence of a vast hydrogen cloud which envelopes and trails the exoplanet as it sweeps around its host star, creating a large comet-like tail in the process. An international team of astronomers, led by David Ehrenreich, a scientific collaborator at the University of Geneva, Switzerland, studied GJ 436b in ultraviolet wavelengths in 2013 and 2014 during multiple transits, with the help of Hubble’s onboard Space Telescope Imaging Spectrograph, or STIS. To their surprise, the researchers noted that approximately two hours before and three hours after every transit of GJ 436b across the face of its host star, the planet would exhibit a distinct absorption line in its spectrum corresponding to atomic hydrogen (a phenomenon that is better known as Lyman-alpha absorption in astrophysics). Following a careful systematic analysis in order to correct for any errors or other artifacts in the data, the researchers came to the conclusion that what they were observing was an immense hydrogen cloud that was surrounding the planet while following it in its orbit, creating a comet-like tail almost 50 times bigger than the planet’s host star.
“We propose that the asymmetric absorption is caused by the passage of a huge hydrogen cloud, surrounding and trailing the planet,” write the researchers in their study, which was published on June 25 in the journal Nature. “The planetary atmosphere is an obvious source for this hydrogen. To produce this extinction signature, we estimate that an ellipsoidal, optically thick cloud of neutral hydrogen should have a projected extension in the plane of the sky of approximately 12 stellar radii along the orbital path of the planet and approximately 2.5 stellar radii in the cross direction, well beyond the planet’s Roche lobe radius.”
The cause for all this gas enveloping GJ 436b stems from the fact the radiation pressure from the planet’s host star isn’t strong enough to drive away all of the escaping atmospheric hydrogen fast enough, thus allowing it to form a vast coma and tail around the planet that follows the latter in its orbit. “You would have to have Hubble’s eyes,” says Ehrenreich. “You would not see it in visible wavelengths. But when you turn the ultraviolet eye of Hubble onto the system, it’s really kind of a transformation, because the planet turns into a monstrous thing.”
The findings by both Hu’s and Ehrenreich’s teams open the door for the potential discovery of more such “warm Neptunes” in the future, which astronomers believe might actually be found all across the galaxy. “We don’t have any planets like this in our own Solar System,” says Hu. “But we think [such] planets with helium atmospheres could be common around other stars.”
“Any planet one can imagine probably exists, out there, somewhere, as long as it fits within the laws of physics and chemistry,” adds Dr. Sara Seager, an astronomer at the Massachusetts Institute of Technology and member of Hu’s team. “Planets are so incredibly diverse in their masses, sizes and orbits that we expect this to extend to exoplanet atmospheres.”
Perhaps even more importantly, the technique that was used by Ehrenreich’s team for the detection of the hydrogen absorption lines in GJ 436b’s spectrum could also be used in the future for the detection of similar spectral features on terrestrial, potentially habitable exoplanets as well, which could be exhibiting a much more temperate atmospheric hydrogen escape due to the very low evaporation of liquid water on their surfaces, similar to the one observed on the seas and oceans of Earth.
As is often the case in the field of exoplanetary research, fascinating new discoveries await as at every turn, slowly bringing us closer to a deeper understanding of the plurality of worlds and potentially life as well, which might exist out there in the cosmic dark.