It would explain a lot if two recent discoveries involving ‘mini-Neptunes’ turned out to be representative of what happens to their entire class. For Michael Zhang (Caltech) and colleagues, in two just published papers, have found that mini-Neptunes can lose gas to their parent star, possibly indicating their transformation into a ‘super-Earth.’ If such changes are common, then we have a path to get from a dense but Neptune-like world to a super-Earth, a planet roughly 1.6 times the size of the Earth and part of a category of worlds we do not see represented in our Solar System.
As we drill down toward finding smaller worlds, we’ve been finding a lot of mini-Neptunes as well as super-Earths, with the former two to four times the size of the Earth. Thus we have a bimodal gap in exoplanet observation. Where are the worlds between 1.6 and 2-4 times the size of Earth? The new work examines two mini-Neptunes around the TESS object TOI 560, located about a hundred light-years from Earth, and a pair of mini-Neptunes orbiting HD 63433, about 70 light years away. At TOI 560, the planets have periods of 6.4 days and 18.9 days; at HD 63433, the periods are 7.1 and 20.5 days.
At both stars we find a planet whose atmosphere is being stripped away, creating a large cocoon of gas. At TOI 560, it is the innermost mini-Neptune that is losing atmosphere; at HD 63433, the process is occurring on the outer world. Zhang, who is lead author of the two papers on this work, speculates that at the latter, the inner world may already have had its atmosphere stripped away; while the signature of hydrogen is found at the outer mini-Neptune, it is not detected at HD 63433 b, the inner world. The paper notes:
The predicted mass-loss timescale for planet c is longer than the age of the system, but the corresponding mass-loss timescale for planet b is significantly shorter. This implies that c could have retained a primordial H/He atmosphere, while b probably did not.
These planets are Neptune-like in having a rocky core surrounded by a thick envelope of what is thought to be hydrogen and helium. Using Hubble and Keck data for TOI 560 and HD 63433 respectively, the scientists found that at least in these systems, hot Neptunes can transform into super-Earths.
A small enough mini-Neptune close enough to its star undergoes atmospheric loss under the bombardment of stellar X-rays and ultraviolet radiation. The remnant world would be smaller in radius, while any planet in the ‘radius gap’ between 1.6 and 2-4 Earth radii would be in transition, in the process of losing much of its atmosphere over a period of hundreds of millions of years.
“Most astronomers suspected that young, small mini-Neptunes must have evaporating atmospheres,” adds Zhang. “But nobody had ever caught one in the process of doing so until now.”
Image: This is an artist’s Illustration of the mini-Neptune TOI 560.01, located 103 light-years away in the Hydra constellation. The planet, which orbits closely to its star, is losing its puffy atmosphere and may ultimately transform into a super-Earth. Credit: Artwork: Adam Makarenko (Keck Observatory).
We have yet to determine whether the process is common, because other scenarios are possible. It is conceivable that some of the mini-Neptunes we observe are actually water worlds that are not enshrouded in hydrogen at all. As the paper on TOI 560 notes:
An alternate explanation for the radius gap is that it has nothing to do with mass loss, but is instead because cores have a broad mass distribution, with the smaller cores having never accreted gas in the first place (Lee & Connors 2021). It is also possible that some mini-Neptunes have no hydrogen-rich envelopes at all, but instead formed with substantial water-rich envelopes (e.g., Mousis et al. 2020). This could dramatically change the mass-loss rates, especially that of helium, which would have been already lost to space alongside the primordial hydrogen.
But TOI 560.01 and HD 63433 c are in the spotlight because they offer the first evidence for the theory that mini-Neptunes do become super-Earths. That evidence is strengthened by the the speed of gasses in their atmospheres. Helium at TOI 560.01 is moving as fast as 20 km/sec, while hydrogen at HD 63433 c reaches 50 km/sec.
These data are the result of transmission spectroscopy, in which light from the star is observed passing through a planetary atmosphere, thus carrying information about its composition and characteristics. The degree of motion here precludes retention by the planet, a fact that is bolstered by the size of the gas cocoons around both worlds. At TOI 560.01, the gas is detected in a radius 3.5 times that of the planet, while at HD 63433 c hydrogen is found at a distance at least twelve times the radius of the planet.
The work on TOI 560.01 involved two transits, both of which showed strong helium absorption and some evidence of variability in the atmospheric outflow. Bear in mind that this is the first mini-Neptune with a helium detection, and given that this system contains two worlds where a potential transformation into a super-Earth is possible, we have a new way to explore what Zhang calls ‘exoplanet demographics.’ From the paper:
TOI 560 is a two-planet system, and TOI 560.02 is also a transiting mini-Neptune. This makes the system an excellent test for mass-loss models. The two planets share the same contemporary X-ray/EUV environment, as well as the same irradiation history. In addition, planets of similar size located in adjacent orbits might be expected to have largely similar formation and/or migration histories, and therefore it is reasonable to expect that their primordial atmospheric compositions would be quite similar. This is supported by observational studies of the masses and radii of multi-planet systems in the Kepler sample, which suggest that planets in the same system tend to have similar masses and radii (the “peas in a pod” theory; Weiss et al. 2018).
An intriguing aspect of the situation at TOI 560 is that the innermost world shows a gas outflow that seems to be moving toward the central star. It will take future observations of other mini-Neptunes to find out just how anomalous this may be.
The papers are Zhang et al., “Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune.” Astronomical Journal Vol. 163 No. 2 (17 January 2022) 68 (full text); and Zhang et al., “Escaping Helium from TOI 560.01, a Young Mini-Neptune,” Astronomical Journal Vol. 163 No. 2 (17 January 2022) 67 (full text).
Is there any possibility that the evaporating atmospheres of hot mini-Neptunes offer an explanation for the earth (and Mars) faint young sun paradox? It has been suggested that a thick atmosphere of h2 could have kept the Earth’s surface warmer and maintained liquid water when it apparently should have been a frozen snowball. Mars’ early water would be an even harder problem during this eon. If Mars also formed with a thick H2 atmosphere that rapidly escaped, would that also have been sufficient to allow liquid water on its surface when the sun was much fainter?
I hadn’t thought of this work in connection with the young sun business, but yes, on reflection, it seems to make sense as a possibility. I’m going to keep my eye out for related work that may connect to young sun ideas. Good thought!
IIRC Earth would have been too small in radius and mass under current modeling to hold on to any meaningful hydrogen in its atmosphere during formation. It’s not until you get above planets about 1.3 times Earth radius and 2-3 times its mass that they start holding on to hydrogen envelopes.
When you say “hold on”, you mean stable. I agree that there cannot be a stable H2 atmosphere. But what about a transient situation where the H2 evaporates off, leaving the CO2-N2 atmosphere. The faint sun will lower the temperature initially, although it would rise again as the H2 warmed the surface. I agree it may be problematic, but I just ask whether there is a possible case to be made for an unstable H2 addition to the atmosphere that disappears over time.
If the Earth were to have an H2 atmosphere of say 10 bar, how long would it take before it was lost?
Alex and Paul,
The possible role of Hydrogen in warming the early Earth
has been considered:
https://www.science.org/doi/epdf/10.1126/science.1225759
The issue isn’t whether a dense H2 atmosphere can solve the needed GHG to keep the early Earth warm, it is whether the H2 envelope can even form and remain long enough. Brett has rightly questioned this speculation.
From the Science paper:
And:
IOW, there is the issue of the residency time of the H2 envelope, which is what the evaporating H2 atmosphere of mini-Neptunes was stimulating as an idea. The authors in the Science paper seem to accept that abiotic H2 production by geology (see my CD posts about geology driving energy sources for life) may have been the responsible source of the gas for their analysis after the primordial H2 was lost. They also suggest that the coupling with N2 lengthened the period of H2 in the atmosphere. It was the emergence of the archaean methanogens that removed this geologic source of H2. [I also note that photolysis of H2O could conceivably have supplied some H2 to the atmosphere, although this would be small and self-limiting as O3 formation would block the UV needed.]
David, if you have a good article on the dynamics of an early Earth atmosphere with H2, especially as regards how long the H2 would remain to maintain the needed warming to allow abiogenesis by 3.8 bn ya, that would be interesting. While abiogenesis could conceivably have occurred in ocean vents kept ice-free by volcanism, we still want a mechanism to prevent the Earth from being a frozen snowball at this time which seems to be at odds with the evidence. Hence the paradox.
FEBRUARY 9, 2022
Final moments of planetary remnants seen for first time
by University of Warwick
https://phys.org/news/2022-02-moments-planetary-remnants.html
Maybe Dark Star was right!
https://centauri-dreams.org/2017/11/19/dark-star-and-staring-into-the-cosmic-abyss/
And just announced: a 3rd planet discovered around Proxima Centauri;
J. P. Faria et al, A candidate short-period sub-Earth orbiting Proxima Centauri, Astronomy & Astrophysics (2022). DOI: 10.1051/0004-6361/202142337
More on this tomorrow.