Mini-Neptunes Turning Into Super-Earths Could Explain Exoplanet Mystery

Astronomers have spotted two mini-Neptune exoplanets that are losing their puffy atmospheres, in the process possibly transforming into super-Earths. Radiation from the planets' parent stars is stripping their atmospheres by boiling away gas, causing it to escape like steam from a boiling pan.

The findings could solve the mystery of why there is a size gap in the planets found outside the solar system between these two types of worlds. The results could give scientists a better picture of how worlds like these evolve.

Caltech graduate student Michael Zhang explained in a press release from W.M. Keck Observatory: "Most astronomers suspected that young, small mini-Neptunes must have evaporating atmospheres. But nobody had ever caught one in the process of doing so until now."

Zhang is the lead author of two papers detailing the team's findings, published in The Astronomical Journal, based on data collected by the Hubble Space Telescope and Hawai'i's W.M. Keck Observatory.

Mini-Neptunes are planets that are smaller denser versions of the solar system ice-planet Neptune. They are composed of a rocky core surrounded by thick gaseous outer layers of hydrogen and helium that are leftover from the formation of their central star. These worlds are usually around two to four times the size of Earth.

Super-Earths, on the other hand, are rocky terrestrial worlds that are between 1.6 to 1.75 times the size of our planet. Astronomers don't detect planets in-between the sizes of these two types of worlds, something that has been a mystery until now.

The puzzle could be solved if mini-Neptunes had their outer layers of gas stripped away by stellar X-rays and ultraviolet radiation, leaving behind a rocky core. This would be classified as a super-Earth with a smaller radius than the original mini-Neptune possessing a thin, tenuous atmosphere.

Professor of Planetary Science Heather Knutson said: "A planet in the gap would have enough atmosphere to puff up its radius, making it intercept more stellar radiation and thereby enabling fast mass loss.

"But the atmosphere is thin enough that it gets lost quickly. This is why a planet wouldn't stay in the gap for long."

That would explain why planets aren't observed in the size gaps between mini-Neptunes and super-Earths. These planets can't hang on to their atmospheres long enough to remain in that size gap.

While this process has been theorized for some time, this is the first time astronomers have found direct evidence of it occurring.

Mini Neptune
An illustration showing the min-Neptune TOI 560.01. Astronomers have observed gas being stripped away from this exoplanet a process that could possibly leave behind a stripped rocky core that becomes a super-Earth. Adam Makarenko/W. M. Keck Observatory

The team's findings delivered another surprise for astronomers, showing gas flowing towards the planet's parent star rather away from it, as would be expected.

To reach these conclusions the team studied two separate mini-Neptunes. The team used the Keck Observatory's Near-Infrared Spectrograph (NIRSPEC) to study one of a pair of mini-Neptunes in TOI 560, a star system found 103 light-years away from Earth. They also used the Hubble Space Telescope to observe two slightly closer mini-Neptunes orbiting the stars HD 63433, located 73 light-years away.

In HD 63433's system, gas was being stripped from the outermost mini-Neptune, designated HD 63433 c. In the innermost mini-Neptune of TOI 560, named TOI 560.01, atmospheric gas was not just leaking from the planet but was unexpectedly traveling towards the system's parent star.

The gas the team observed escaping from the planets was doing so extremely rapidly, at around 12.5 miles per second for TOI 560.01, and as fast as 31 miles per second for HD 63433 c. That means that in both cases the gas is moving way too fast for the gravity of these planets to hang on to it.

The team will now attempt to discover if the phenomena of gas flowing towards the central star is unique to TOI 560.01, or if this is replicated around other mini-Neptunes.

Knutson said: "We still have a lot to learn about how these outflows work in practice.

"As exoplanet scientists, we've learned to expect the unexpected. These exotic worlds are constantly surprising us with new physics that goes beyond what we observe in our solar system."

Mini Neptune and Super Earth
(Right) an illustration of a mini-Neptune have its atmosphere stripped. (Left) A super Earth. Astronomers have observed atmospheres being stripped from mini-Neptunes, a process that could transform them into super-Earths. W.M Keck/NASA