Watching the snowline descend to ever lower elevations as fall deepens into winter is one of the great pleasures of the Canadian Rockies, an area better suited to train travel than any on Earth. And an image of snow-topped mountains in Alberta came back to me as soon as I read about another kind of snowline, the boundary between the inner regions of a solar system, where rocky planets tend to form, and the outer depths, which become the domain of cold, gaseous worlds. The snowline holds clues to how ‘super-Earth’ planets form.
The paper, by Grant Kennedy (Mt. Stromlo Observatory, Australia) and colleagues, takes a hard look at M-class red dwarfs and contrasts them to solar-type stars. The latter show a relatively constant luminosity during planet formation, meaning conditions change little during this era. But red dwarfs fade dramatically as they evolve toward maturity, dimming to the point where what had been a warm inner disk begins to freeze. And that has implications for planetesimals forming in the disk, especially the so-called ‘super-Earths.’ Thus Kennedy:
“It’s like a massive cold front that sweeps inward toward the star. The ices add mass to a growing planet, and also make it easier for particles to stick together. The two effects combine to produce a planet several times the size of Earth.”
Microlensing observations suggest that planets in the range of five to fifteen Earth masses might be common around M-class dwarfs, normally occurring between 2.5 and 3 AU from their parent star. The assumption is that these are ice giants like Uranus and Neptune in our own Solar System. And what Kennedy and team are proposing is that they form out of the relatively meager disk material around red dwarfs, emerging from snowstorms lasting millions of years, storms that envelop and help to create the resulting planet.
But bear in mind the distances involved when you hear the term ‘super-Earth.’ These would be icy planets with no liquid water orbiting far out of a red dwarf’s habitable zone. In fact, little planet forming material seems to exist close in to an M-dwarf, leading to the supposition that planets larger than Mercury or Mars would be unlikely to emerge there. The good news is that a Mars-size planet is perfectly suitable for the development of life in such a close-in orbit.
The paper is Kennedy et al., “Planet formation around low mass stars: the moving snow line and super-Earths.” It’s slated for publication in the The Astrophysical Journal Letters, but the preprint is already available.
And here’s Centauri Dreams‘ take: Are we certain that the protoplanetary disks around red dwarfs contain proportionately less material than those around Solar-type stars? It seems a reasonable assumption, but any play in the numbers there could drastically effect what we’ll expect to find one day around stars like Proxima Centauri or Barnard’s Star. Nailing down those numbers will be a lengthy and perhaps controversial process, and the recent discovery around GJ 849 makes the outcome look more problematic than it did a few months ago.
Hi Paul
Mercury or Mars sized planets can retain atmospheres, but whether they can sustain geology is another matter. I think they’re likely to be barren after a few billion years due to solar wind erosion of nitrogen etc. especially after their magnetic-field dynamo sputters out. Thus good terraforming sites, but unlikely to support a home planet of ETIs.
Adam
Good point re the solar wind, and interesting because the usual caveat about M dwarfs is their flare activity.
There might also be erosion from high velocity impacts, since meteoroids would strike the planet at higher speed than they would our own.
Hey I had totally forgotten that one. Up to 90% of Mars’s original atmosphere is believed to have been lost via impactors. Orbitting a 0.25 solar mass M dwarf at 0.05 AU enhances impact energy by more than a factor of 5 since the planet’s orbital velocity is higher too. I wonder if the frequency of impactors would be lower though due to the difficulty in actually getting them in so close to the star. There are lots of sun-grazer comets though so comet impact rates could well be higher.
There’s another question – what are the comet clouds of M dwarfs like? Are they more compact because the ice giants are relatively closer in and there’s no big Jovians to give them a big kick? If so such clouds should be more stable against near-misses by other stars, though perhaps more unstable against galactic tides. Wow! What a lot of new questions about M dwarf systems.