A Closer Look at a Potential "Eyeball Planet"

Imagine a world hospitable to life, with a single temperate ocean surrounded on all sides by ice. This “eyeball planet” might sound straight out of science fiction, but it is entirely possible — and astronomers think they might have found such a world in LHS 1140b.

Located only 49 light-years away in the constellation Cetus, LHS 1140b is one of the closest discovered planets that lies within its star’s habitable zone — the region where a planet could retain liquid water.

In a paper published in Astrophysical Journal Letters, researchers analyzed atmospheric data from the James Webb Space Telescope’s Near Infrared Imager and Slitless Spectrograph, finding hints that LHS 1140b could host a nitrogen-rich atmosphere.

Added to the team’s previously published conclusions that this world might be made of water ice, the hints of nitrogen-based air would allow for oceans on this planet’s surface. It’s also possible, because LHS 1140b always faces the same side toward its star, that the world could be largely icy, with a temperate ocean only on its dayside — making it look like an “eyeball planet.”  

However, the team only has hints about the atmosphere’s composition so far — the researchers couldn’t rule out that the planet might have no atmosphere at all, making it a world of barren rock or ice.

“What I find to be the most significant about this result is that there's an indication that LHS 1140b might have an atmosphere at all,” says Jason Dittmann (University of Florida), a co-discoverer of LHS 1140b who was not involved in this study.

LHS 1140b is the first rocky exoplanet to have shown hints of an atmosphere, and while the study’s atmospheric results are tentative, LHS 1140b is certainly a promising habitable candidate to keep an eye on.

Hints of an Atmosphere

At a radius around 1.7 times greater than Earth’s, LHS 1140b is a rare find. While most super-Earths are less than 1.5 times Earth’s radius, and most mini-Neptunes have radii between 2 and 3 times Earth’s, LHS 1140b is one of the few planets found so far that falls in between, in the so-called “radius valley.”

Based on its mass and radius, the team previously found that LHS 1140b’s average density indicates it’s either a mini-Neptune encased in a thin envelope of hydrogen, or it’s a “water world” with water making up to a fifth of its composition. (Comparatively, our own planet’s mass is only 0.02% water. ) Such a world might have a thin envelope of nitrogen or carbon dioxide.

To figure out which option best describes LHS 1140b, the astronomers used Webb to measure the planet’s atmosphere. As LHS 1140b makes its 25-day transit in front of its host star, light from the star passes through the planet’s atmosphere — if it has one — like a lantern shining through fog. Molecules in the planet’s upper atmosphere will absorb the star’s light at different wavelengths, making fingerprints in the stellar spectrum.

The astronomers found no evidence of atmospheric hydrogen — backing up previous observations taken with several other space telescopes. The researchers thus reject a cloudless, hydrogen-rich mini-Neptune scenario. The team concludes that the planet is a water world instead; hints of nitrogen in LHS 1140b’s atmosphere align with this conclusion.

However, the study also found that the Webb spectrum, which is mostly flat and featureless, could allow for no atmosphere, especially when considering the star’s own activity. Hot active regions on the host star called faculae can add to the light that shines through the planet’s atmosphere. “A fit with only stellar faculae (and no planetary atmosphere) provided a reasonable fit [to the data],” says team member Ryan MacDonald (University of Michigan).

Nevertheless, a model that combined stellar faculae and a nitrogen-rich atmosphere better matched the Webb data. Other factors also support an atmosphere for LHS 1140b: It’s more massive than Earth, so has a strong surface gravity that can better hold onto atmosphere. Its host star is also less active than others (such as TRAPPIST-1, whose radiation might have stripped the atmospheres of its rocky planets).

To make the most of the Webb data, the team also used global climate models to simulate interactions between different possible components of a planet’s climate – including not only its atmosphere, but also any land surface, ocean, and sea ice. These simulations can help assess the likelihood of different atmospheres as well as the possibility of liquid oceans.

“We actually have 3D global climate simulations of the planet that predict such liquid ocean and clouds only on a small region,” Cadieux said. “LHS 1140 b is the most credible ‘eyeball planet’ candidate we know.”

Next Steps

The team hopes to follow up with the Webb Telescope to determine the planet’s surface characteristics and delve deeper into its atmosphere. It will likely take years to obtain all the needed data.

“Our tentative result of a nitrogen-rich atmosphere on LHS 1140b, if firmly confirmed by additional observations, would be the first such detection,” Cadieux says. Currently, the only planets in their host star’s habitable zone with a definitive detection of an atmosphere are Earth and Mars.

While Webb’s spectrum of LHS 1140b turned up only hints of nitrogen, Dittman says those hints warrant more telescope time. “More data might confirm it or it might make the result disappear,” he says. “Right now, it's an exciting time for exoplanets!”