The Case of the Helium-Shrouded Exoplanet With the Hydrogen, Comet-Like Tail

An artist's concept of the vast envelope of atomic hydrogen that envelopes the nearby helium-dominated exoplanet Gliese 436b. The radiation pressure from the planet's host star causes the gradual escape over time of significant amounts of hydrogen from Gliese 436b's atmosphere into space, which subsequently forms a large comet-like tail that follows the planet in its orbit around its star. Image Credit: NASA, ESA, and G. Bacon (STScI)

An artist’s concept of the vast envelope of atomic hydrogen that envelopes the nearby helium-dominated exoplanet Gliese 436b. The radiation pressure from the planet’s host star causes the gradual escape over time of significant amounts of hydrogen from Gliese 436b’s atmosphere into space, which subsequently forms a large comet-like tail that follows the planet in its orbit around its star. Image Credit: NASA, ESA, and G. Bacon (STScI)

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.

This diagram illustrates the process with which an exoplanet like Gliese 436b might transition from an initial hydrogen-helium atmosphere, to a helium-dominated one in the course of several billion years. Intense stellar irradiation causes a rapid evaporation of more than 10 percent of atmospheric hydrogen during the first few hundred million years of the planet's life (middle), eventually leaving behind a helium-dominated atmosphere with only traces of carbon monoxide (bottom). Image Credit: NASA/JPL-Caltech

This diagram illustrates the process with which an exoplanet like Gliese 436b might transition from an initial hydrogen-helium atmosphere, to a helium-dominated one in the course of several billion years. Intense stellar irradiation causes a rapid evaporation of more than 10 percent of atmospheric hydrogen during the first few hundred million years of the planet’s life (middle), eventually leaving behind a helium-dominated atmosphere with only traces of carbon monoxide (bottom). Image Credit: NASA/JPL-Caltech

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.”

Artist's concept of the exoplanet Gliese 436b as it might look like to our eyes. Since the planet's atmospheric hydrogen is being depleted with time leaving behind a helium-dominated atmosphere, scientists predict that Gliese 436b would appear covered in white or gray clouds. Image Credit: NASA/JPL-Caltech

Artist’s concept of the exoplanet Gliese 436b as it might look like to our eyes. Since the planet’s atmospheric hydrogen is being depleted with time leaving behind a helium-dominated atmosphere, scientists predict that Gliese 436b would appear covered in white or gray clouds. Image Credit: NASA/JPL-Caltech

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.

 

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3 comments to The Case of the Helium-Shrouded Exoplanet With the Hydrogen, Comet-Like Tail

  • karol

    “fascinating discoveries await us at every turn, slowly bringing us to a deeper understanding of the plurality of worlds and potentially life as well, that might exist out there in the cosmic dark.” What an excellent summation of an exceptionally well-written article. I always know that when I see the byline “Leonidas Papadopoulos” that I am in for an engrossing, exciting “Sagan-style” exploration of the cosmos which is a great complement to the exceptional Ben and Mike “you are (were) there works. Your articles involve complex issues of astrophysics, and are without question well worth the time to slowly, carefully study and digest your in-depth research. Bravo Leonidas, another EXCELLENT work of research and writing! Off topic, but I don’t give a damn, so if you NewSpace howler monkeys don’t like it, just move on. Leonidas my friend, I just saw on the NBC news that the economic situation in Greece is no better, and probably growing worse. When people’s children get hungry, they start getting desperate – and dangerous (Hey, I guess I’m hardly one to talk. Here in Detroit in what passes for our “Greektown”, recently one young man as shot dead, and in another incident a young woman was wounded). I know you are loyal to your family and friends, but the United States could certainly use someone of your impressive intellectual ability which is far greater than that of the average American. Perhaps the Space Command of the U.S. Air Force would be a path to citizenship and an opportunity to gain so “hands on” aerospace experience. They would be lucky to have someone with your gifts in research and writing. A collection of your articles would make a very impressive resume. Whatever happens in your beautiful country, I wish you all the best Leonidas, and for God’s sake, keep your head down – we can’t afford to lose you!

  • Leonidas Papadopoulos

    I’d also like to truly thank you Karol, for your wonderful, moving and always inspiring commentary. One cannot ask for better friends than you!

    As for Greece, yes, things are worse than bad right now. Banks are on a capital control status, the natives are growing restless and the country faces the risk of an imminent and deafening collapse. As for me, I’m keeping a surprisingly calm and optimistic outlook, to the point that many people complain to me that I act as if I’m on a different planet. That couldn’t be further from the truth however. I deeply care about these things, but having gone through much fear, stress and anxiety about it for the last couple of years, I realised that you can only deal effectively with the situations only if you try to be positive and negative-free.

    As for your suggestion about immigrating abroad, it’s easier said than done from a procedures point of view…

    My best regards as always Karol!

    Leonidas

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