A Gas Giant Was Transformed Into an Earth-Sized Core With a Planetary Atmosphere

Planets that tread too close to host stars get second chances, after all.
Brad Bergan

Scientists using NASA's Hubble Space Telescope just found evidence that an intriguing, rocky exoplanet called GJ 1132 b may have transformed from a gas giant into an Earth-sized core, and then amassed a second, planetary atmosphere from volcanic activity — according to a recent study shared on a preprint website.

This is the first detection of a "secondary atmosphere" beyond our solar system — and it could have substantial ramifications for other exoplanets.

Hot magma gave this Earth-sized planet a second chance

The planet orbits a red dwarf star 41 light-years from Earth, and — while it shows notable parallels to our planet — the differences are more gripping. This is a planet with a smoggy, hazy atmosphere filled with a toxic cocktail of hydrogen, methane, and hydrogen cyanide.

Yes, cyanide.

Scientists used NASA's Hubble Space Telescope to uncover evidence that this planet's current atmosphere was not its 'first time.' The original atmosphere was initially blasted away from deadly radiation from GJ 1132 B's parent red-dwarf star.

The planet's second atmosphere is thought to have formed from volcanism — specifically, as molten lava under the surface continually oozes up through volcanic fissures, gases firing through the cracks serve as a consistent source of replenishment to the planet's "secondary atmosphere," without which this one, too, would be stripped away by the host star.

Planets dangerously close to host red dwarf stars can regenerate atmospheres

Scientists think the distant exoplanet began as a gas world — smaller than Neptune, but several times the diameter of Earth and awash in a primordial hydrogen-helium atmosphere. But the initial atmosphere was lost due to its hot and young host star — which stripped the "sub-Neptune" planet down to its Earth-sized core in a short time.

The scientists based this analysis on a combination of direct observational evidence and inference via computer modeling — which suggested the aerosol haze is similar to photochemically produced hydrocarbons: what we call smog on Earth.

"It's super exciting because we believe the atmosphere that we see now was regenerated, so it could be a secondary atmosphere," said study co-author Raissa Estrela at NASA's Jet Propulsion Laboratory (JPL) in Southern California, according to a Phys.org report. "We first thought that these highly irradiated planets could be pretty boring because we believed that they lost their atmospheres. But we looked at existing observations of this planet with Hubble and said, 'Oh no, there is an atmosphere there.'"

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These findings — which will be published in a forthcoming issue of The Astronomical Journal —could transform our expectations for planets outside of our solar system.

"How many terrestrial planets don't begin as terrestrials? Some may start as sub-Neptunes, and they become terrestrials through a mechanism that photo-evaporates the primordial atmosphere," said the study's lead author Mark Swain of JPL, in the Phys.org report. "This process works early in a planet's life, when the star is hotter."

Once the star cools off, the planet is left there, in whatever final state it's reached, explained Swain. "So you've got this mechanism where you can cook off the atmosphere in the first 100 million years, and then things settle down. And if you can regenerate the atmosphere, maybe you can keep it."

GJ 1132 b is tidally-locked with its host star, like the moon to Earth

Obviously, the toxic mix of GJ 1132 b's atmosphere makes it grossly uninhabitable for humans. But it's also so close to its host red-dwarf star that it executes an orbit in just 1.5 days, and is even tidally locked — with the same face of the surface to the star forever, just like the moon's "face" faces us.

"The question is, what is keeping the mantle hot enough to remain liquid and power volcanism?" asks Swain in the report. "This system is special because it has the opportunity for quite a lot of tidal heating."

Tidal heating is what happens when a rocky body is squeezed and stretched by its host body during orbit — similar to Jupiter's volcanically-active moon, Io — resulting in a "pumping" action that heats the interior of the planet for a long time.

While GJ 1132 b is not friendly to human life, it has a lot to teach us about how exoplanets evolve — especially around red dwarf stars, which are thought to be the most abundant in our galaxy. And, when NASA's James Webb Space Telescope begins its mission, it could give scientists a glimpse of the planet's surface. "If there are magma pools or volcanism going on, those areas will be hotter," said Swain. "That will generate more emission, and so they'll be looking potentially at the actual geologic activity — which is exciting!" It is.

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