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rfmcdonaldThe most common stars and star-like objects in the universe are dim, since it's easier to accumulate less mass. Our Sol actually isn't a typical star at all, more massive and bright than more than 95% of its local peers. The red dwarf is a star that's barely massive enough to huge hydrogen at all; the brown dwarf is less massive still, glowing briefly in its youth but then fading.
Could life--that is to say, Earth-like life--exist on a world orbiting these stars? The traditional scientific response has been skeptical, but this has been changing of late. Centauri Dreams had the news, noting that things actually look reasonably good for red dwarfs.
Red dwarfs won't irradiate their worlds: “Throughout most of the flare, the surface of our model Earth-like planet experienced no more radiation than is typical on a sunny day here on Earth.” AD Leonis, Gilster notes, was chosen because it was so active a star. Although the effect of charged particles and multiple flares in rapid secession has to be taken into account, red dwarfs seem safe.
Brown dwarfs look acceptable, too, although the question with brown dwarf was a bit of a non-question (could they form planets at all?).
Gilster quotes an interesting scenario from andy.
Could life--that is to say, Earth-like life--exist on a world orbiting these stars? The traditional scientific response has been skeptical, but this has been changing of late. Centauri Dreams had the news, noting that things actually look reasonably good for red dwarfs.
[I]n recent times the rap against M-dwarf planets has been that their stars are prone to violent convulsions that launch potentially lethal flares into their planetary systems. Many M-dwarfs produce high energy charged particles and short-wavelength radiation from X-rays to ultraviolet. All of this activity can also affect a planet’s atmosphere, so that a key question becomes whether a planet in an M-dwarf’s habitable zone can retain its atmosphere, or whether terrestrial worlds would lose hydrogen and helium and gas giants would erode into Neptune-mass cores.
My friend and I kicked this around before parting company, he returning to studies unrelated to astronomy, while I returned to my office to find a message from Adam Crowl on red dwarfs and flare activity. A new study demonstrates that red dwarf planets may be shielded from these flares after all. As is standard practice in these matters, Antigona Sugura (Universidad Nacional Autónoma de México) and team went to work with computer models, simulating how a 1985 flare from the star AD Leonis would have affected an Earth-like planet orbiting it at 0.16 AU. AD Leonis is an M-dwarf about 16 light years from Earth, and 0.16 AU, about half Mercury’s distance from the Sun, is in the zone where liquid water could exist at the surface.
The results are promising. It turns out that in the simulation, bursts of UV radiation hitting an Earth-like atmosphere produced a thicker ozone layer, protecting the surface.
Red dwarfs won't irradiate their worlds: “Throughout most of the flare, the surface of our model Earth-like planet experienced no more radiation than is typical on a sunny day here on Earth.” AD Leonis, Gilster notes, was chosen because it was so active a star. Although the effect of charged particles and multiple flares in rapid secession has to be taken into account, red dwarfs seem safe.
Brown dwarfs look acceptable, too, although the question with brown dwarf was a bit of a non-question (could they form planets at all?).
We can find suggestive analogs to planet formation around brown dwarfs in nearby space. The star Gl 876, some fifteen light years away, is not a brown dwarf, but this M-dwarf is only 1.24 percent as luminous as the Sun, with most of its energy being released at infrared wavelengths. We now know that at least three planets, two of them gas giants similar to Jupiter, orbit the star. Among brown dwarfs themselves, we have cases like 2M1207b, MOA-2007-BLG-192Lb and 2MASS J044144. In fact, the planet orbiting the second of these brown dwarfs is one of the smallest exoplanets known at 3.3 Earth masses.
As Andrey Andreeschchev and John Scalo (University of Texas) noted in a 2002 paper (thanks to Centauri Dreams regular ‘andy’ for the tip), we can extrapolate from what we find in our own Solar System to lower-mass stars, with simulations indicating that terrestrial-mass planets can form around low-mass objects like these as long as sufficient disk material is available. The authors study whether or not such planets can be habitable, noting this key fact about brown dwarf evolution: The brown dwarf is continually fading as it releases gravitational potential energy. As the object fades, its habitable zone moves past any worlds in it.
Gilster quotes an interesting scenario from andy.
It’d be interesting to come up with some scenarios for evolution on such a planet whose star decreases in luminosity as it ages (as opposed to more conventional stars that brighten as they age) – perhaps life might begin in the cloud layers of an initially Venus-like planet, moving to the surface as the atmosphere cools and the oceans rain out of the atmosphere, and finally moving to a more Europa-like state with the oceans frozen under an ice layer.
