Can one type of planet become another? Can a mini-Neptune lose its atmosphere and become a super-Earth? Astronomers have found two examples of mini-Neptunes transitioning to super-Earths, and the discovery might help explain a noted “gap” in the size distribution of exoplanets.
It’s only natural that we classify exoplanets in ways related to our Solar System’s planets. We use the terms Hot Jupiters, mini-Neptunes, and Super-Earths because they help us quickly recognize what astronomers are talking about. Why do astronomers find so many of these planets in other solar systems rather than planets similar to our Solar System?
That question highlights what’s called the “size gap” in exoplanets.
Two new studies examine a pair of mini-Neptunes that are losing their atmospheres. The discovery not only provides examples of a phenomenon that scientists have theorized about, but it might also help explain the gap in the sizes of exoplanets we find.
The two mini-Neptunes are HD 63433c and TOI 560.01. Each one is the subject of separate papers involving some of the same authors. Michael Zhang is the lead author of both papers: “Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune,” and “Escaping Helium from TOI 560.01, a Young Mini-Neptune.” Both papers are in The Astronomical Journal.
The researchers used the Hubble Space Telescope and the W.M. Keck Observatory to study the planets. They found that both of these mini-Neptunes are undergoing similar transformations. Radiation from their stars strips away their atmospheres and drives the gases into space. The observations suggest that the planets are slowly transitioning into super-Earths.
Mini-Neptunes are planets like Neptune but less massive. Like Neptune, they have thick atmospheres of hydrogen and helium. There are probably deeper layers of ice, rock, maybe even liquid oceans. They have rocky cores and are between 1.7 and 3.9 Earth radii. Mini-Neptunes are also called gas dwarfs.
Mini Neptunian planets range in size from about 1.5 to 4 times the size of Earth and have a rocky core and puffy gaseous shell of varying thickness. Credit: Geoff Marcy
Super-Earths are terrestrial planets larger than Earth but smaller than our Solar System’s ice giants, Uranus and Neptune. Super-Earths have radii as large as 1.6 times Earth’s radius. The term “super-Earth” does not indicate a planet’s potential habitability, only of its size.
This illustration compares Earth and Neptune to the super-Earth CoRoT-7b (center.) Image Credit: By Aldaron, a.k.a. Aldaron – Own work, incorporating public domain images for reference planets (see below), inspired by Thingg’s size comparison, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8854176
One way of looking at these names is that mini-Neptunes are just planets on the upper end of the size scale for super-Earths.
Both these terms became common as we found more and more exoplanets. Sometimes they’re used interchangeably, depending on the context. NASA’s exoplanet catalogue contains 4933 confirmed exoplanet discoveries, and 1723 of them are “Neptune-like,” while 1538 are “super-Earth’s.”
This is a screenshot from NASA’s exoplanet catalogue from February 23rd, 2022. This chart tracks the current number of known planet discoveries beyond our solar system, sorted by type. Image Credit: NASA.
The new papers present examples of mini-Neptunes losing their atmospheres and sliding down the size scale to potentially become super-Earths. Astronomers predicted this phenomenon long ago, but observing it in action was challenging.
“Most astronomers suspected that young, mini-Neptunes must have evaporating atmospheres,” said Michael Zhang, lead author of both studies and a graduate student at Caltech. “But nobody had ever caught one in the process of doing so until now,” he said in a press release.
One of the mini-Neptunes is HD 63433c. It orbits a star about 73 light-years away and 2.67 times Earth’s radius. It’s on an 18.8-day orbit around a star similar to our Sun, but much younger at about 440 million years old. According to the paper, HD 53433c has already lost most of its primordial atmosphere.
The second mini-Neptune undergoing mass-loss is TOI 560.01 (also known as HD 73583b.) Its radius is 2.8 Earth radii. It also orbits a young star, in this case, a 600 million-year-old K-dwarf. Its orbital period is 6.4 days.
This is an artist’s conception of a mini-Neptune or “gas dwarf.” These two new studies present observational evidence that mini-Neptune’s lose portions of their atmosphere due to photoevaporation and transition to super-Earths. Image Credit: By Pablo Carlos Budassi – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=112487881
There’s an evident size gap in the range of exoplanets discovered so far. Most exoplanets are in the super-Earth to the mini-Neptune range, but the range is not populated evenly. Super-Earths can be as large as 1.6 times Earth’s size. Mini-Neptunes are between 2 and 4 times the size of Earth. Astronomers have discovered very few exoplanets with sizes between 1.6 and 2 Earth radii. The few that have been found mean it’s more of a size “valley” than a “gap.” It’s also called the small planet radius gap, the Fulton gap, the photoevaporation valley, or the Sub-Neptune Desert. But whatever we want to call it, it’s there.
Why?
This figure is a histogram of planets with given radii from a sample of 900 Kepler systems. The decreased occurrence rate between 1.5 and 2.0 Earth radii is apparent. Image Credit: Fulton et al. 2017
Exoplanet scientists call the low number of planets between about 1.5 and 2 times Earth’s radius the photoevaporation valley because photoevaporation is behind it. Evidence for the photoevaporation goes back years, and many papers have covered it, though these two new papers are the first to present direct observational evidence supporting it.
Both mass-losing mini-Neptunes are in young solar systems, which helps explain the photoevaporation valley. Theory shows that these planets are shrouded in an atmosphere of hydrogen and helium left over after their star formed. Our own Neptune is far from the Sun, but if a smaller mini-Neptune was closer to its star, then the star could strip away the hydrogen and helium. The atmospheric stripping wouldn’t take long, and these studies show it happening in young solar systems. The photoevaporation can leave behind the mini-Neptune’s rocky core, now a super-Earth. The super-Earth could retain some of its atmosphere depending on the characteristics of the star, the planet, and their separation. The fact that it happens in only hundreds of millions of years would explain the observed photoevaporation valley.
“A planet in the size gap would have enough atmosphere to puff up its radius, making it intercept more stellar radiation and thereby enabling fast mass loss,” said Zhang. “But the atmosphere is thin enough that it gets lost quickly. This is why a planet wouldn’t stay in the gap for long.”
In this artist’s illustration, the small planet transiting close to its star is Kepler-19b. Kepler-19b is a mini-Neptune orbiting a star about 700 light-years away. Kepler 19b has lost much of its atmosphere but is massive enough that it’s retained about 50% of its original atmosphere. Other mini-Neptunes aren’t so massive and can lose much greater percentages of their atmospheres. The larger planet in the foreground is Kepler-19c. Credit: David A. Aguilar (CfA)
Studying the photoevaporative removal of a planet’s atmosphere in a solar system tens of light-years away or more takes away some of the impact. The forces involved in stripping away an atmosphere are epic. “The speed of the gases provides the evidence that the atmospheres are escaping. The observed helium around TOI 560.01 is moving as fast as 20 kilometres per second,” Zhang said. 20 km/s is equal to 72,000 kilometres per hour. For comparison, the International Space Station travels at 28,000 km/h.
Zhang also said the hydrogen escaping from HD 63433c moves at speeds up to 50 kilometres per second. That’s 180,000 kilometres per hour.
“The gravity of these mini-Neptunes is not strong enough to hold on to such fast-moving gas,” Zhang said.
It’s not only the measured speed of the helium and hydrogen around these planets that indicate an escaping atmosphere; it’s the size of their atmospheric envelopes.
“The extent of the outflows around the planets also indicates escaping atmospheres; the cocoon of gas around TOI 560.01 is at least 3.5 times as large as the radius of the planet, and the cocoon around HD 63433c is at least 12 times the radius of the planet.”
If this were happening in our Solar System, what would it look like? The Sun’s stellar wind has battered the Earth for billions of years. We’re only here because Earth’s magnetosphere deflects enough of the Sun’s radiation to hold onto the atmosphere and protect it from photoevaporation. It’s not the same because Earth’s atmosphere is not remnant hydrogen and helium from the solar nebula. So Earth never had a massive gaseous envelope as mini-Neptunes do.
The two mini-Neptunes in the papers have orbital periods of only 18.8 days and 6.4 days. They’re very close to their stars, which makes them great observational targets. What would it be like if our Solar System was home to one of these mini-Neptunes losing its atmosphere?
Imagine a planet with a radius several times larger than Earth, closer to the Sun than Mercury is. It would transit in front of the Sun every week or month. It would be one of the brightest objects in the sky, and periodically the Earth-Moon system might travel through the stream of hydrogen/helium escaping from its atmosphere. It would be something to behold and would’ve shaped all of humanity’s early myths. A massive planet in that location could’ve completely reshaped the inner Solar System’s history.
Some scientists refer to these mass-losing exoplanets as “gas-rich adolescents.” But if they’re adolescents, they’re not the type of adolescents that can become more like Earth. Earth’s atmosphere has a high molecular weight. Like a mini-Neptune, its atmosphere isn’t remnant hydrogen and helium from the solar nebula. Earth’s atmosphere likely has a combination of causes: volcanic out-gassing, loss of lighter atmospheric components to space, even plate tectonics, and magma ocean crystallization on the very young Earth. So finding mini-Neptunes in distant solar systems doesn’t tell us much about how many Earth-like planets might be out there.
This study brings observational evidence to support a long-standing theory. But it also holds a surprise.
The gas escaping from TOI 560.01 moves toward its star rather than away. Is that an anomaly? Exoplanet scientists aren’t sure what might constitute “normal” in many parts of their field. They’ve discovered many oddball planets that don’t look like anything in our Solar System. As far as gas flowing from a mini-Neptune’s atmosphere towards its star, only observations of more transitioning planets can explain if that’s an anomaly or not.
This artist’s image shows Roche-lobe overflow between a mini-Neptune and its star. Image Credit: NASA/GSFC/Frank Reddy
“This was unexpected, as most models predict that the gas should flow away from the star,” said professor of planetary science Heather Knutson of Caltech, Zhang’s advisor and a co-author of the study. “We still have a lot to learn about how these outflows work in practice.”
“As exoplanet scientists, we’ve learned to expect the unexpected,” Knutson said. “These exotic worlds are constantly surprising us with new physics that goes beyond what we observe in our solar system.”