An object we thought belonged to the most common category of planet in the galaxy has turned out to be something we’ve never seen before.
The exoplanet Enaiposha, or GJ 1214 b, is a hazy world orbiting a red dwarf star about 47 light-years from Earth. Previously likened to a mini-Neptune, in-depth observations obtained using JWST now suggest the exoplanet is more like Venus – only much larger.
This would make it the first known of its kind, a category astronomers are calling ‘Super-Venus’.
Enaiposha is one of the most studied exoplanets in the sky. It was discovered in 2009, with a mass and radius that put it somewhere between Earth and Neptune. Subsequent observations revealed a substantial atmosphere.
Exoplanets in this mass regime generally fall into one of two categories. The Super-Earths are thought to be terrestrial exoplanets larger than Earth, hosting hydrogen-rich atmospheres, if they have one at all.
So-called mini-Neptunes can be of a similar size, but their composition is significantly different, with denser atmospheres rich in hydrogen and helium, and liquid oceans possibly wrapping their surfaces. Mini-Neptunes are the most numerous of the more than 5,800 confirmed exoplanets at time of writing, which is interesting, because we have nothing directly analogous to them here in our Solar System.
Both super-Earths and mini-Neptunes are intriguing to scientists because, if other conditions are just right, they might be habitable to life as we know it. This is partly why astronomers closely study Enaiposha, a world clocking in at 2.7 times the radius and 8.2 times the mass of Earth.
Although the world is far too close to its host star, Orkaria, and therefore far too hot to be likely habitable itself, it’s proximity to Earth means we can see it relatively easily, meaning it could give us information that might aid in our understanding other similar exoplanets elsewhere in the galaxy.
But Enaiposha poses some problems, too. Its atmosphere is so thick that we can’t peer into it very easily. But a paper published in 2023 based on JWST and Hubble observations found that the exoplanet may have a water-rich atmosphere that also contains vaporized metals.
Now, new research efforts suggest that we may have missed something. Led by astronomers Everett Schlawin at the University of Arizona and Kazumasa Ohno at the National Astronomical Observatory of Japan, a team of researchers has studied transit data for Enaiposha, and discovered something unexpected.
As the exoplanet passed in front of the star, whipping around on its 1.6-day orbit, JWST data suggests that starlight traveling through Enapoisha’s atmosphere was altered by carbon dioxide, in concentrations similar to the carbon dioxide that makes up more than 96 percent of the atmosphere of Venus.
But the signal was very faint.
“The detected CO2 signal from the first study is tiny, and so it required careful statistical analysis to ensure that it is real,” Ohno says. “At the same time, we needed the physical and chemical insights to extract the true nature of GJ 1214 b’s atmosphere.”
So, in a second paper, the researchers set about conducting theoretical models that could explain the data. The scenario that best fit the observations, they found, is if Enaiposha has an atmosphere dominated by metals at lower altitudes, and only relatively small amounts of hydrogen.
At higher altitudes, the atmosphere consists of a haze dense with aerosols, as well as the CO2 their reading implied. From this emerges the notion of the super-Venus, a world similar to Venus: very hot, and choked by a carbon-rich atmosphere through which it is difficult to see.
But the exoplanet’s trickiness has not yet been circumvented. The observed signature is so small that extensive follow-up will be required to determine if the team’s conclusions are correct, especially because it’s something new.
“We stress the importance of high precision follow-up observations to confirm the metal-dominated atmosphere,” the researchers write, “as it challenges the conventional understanding of interior structure and evolution of sub-Neptunes.”
The research has been published in two papers in The Astrophysical Journal Letters. They can be found here and here.
