What we know about the planets circling M-class dwarf stars is changing rapidly. Recent microlensing surveys have revealed the existence of two ‘super-Earths’ — rocky worlds 5.5 and 13 times as massive as the Earth — around distant red dwarfs. Microlensing has also produced two gas giants around such stars. And radial velocity surveys have found systems like Gl 876, an M-class star orbited by an outer pair of gas giants and an inner super-Earth. Other radial velocity catches are Gl 436 and Gl 581, each accompanied by a super-Earth in a short-period orbit.

A curious fact emerging from these studies is that the frequency of gas giants around M dwarfs seems to be lower than around F, G and K-type stars. In a new paper, Alan Boss (Carnegie Institute of Washington) discusses the formation of these planetary types, arguing that disk instability rather than core accretion may be the cause of their formation. An additional, and in his view critical, factor: the loss of planetary gas envelopes because of strong sources of UV radiation.

More about disk instability in a future post. For now, let’s focus on that radiation. An M star forming in a region of future low-mass star formation like Taurus or Ophiuchus would, in Boss’ model, be expected to be accompanied by gas giant planets, but one originating in a high-mass star formation region would be a different story. From the paper (internal references omitted):

…most stars are formed in regions of high-mass star formation, similar to the Orion and Eta Carina nebulae, where protoplanetary disks are subjected to a withering ?ux of FUV/EUV radiation from the nearby O stars. In the Eta Carina nebula, FUV/EUV ?uxes are a factor of ~ 100 times higher than in Orion, yet protoplanetary disks are as commonplace in Carina as in Orion.

In the quote above, FUV and EUV stand for far ultraviolet and extreme ultraviolet, markers of wavelength. And the effects of such radiation should be readily observable, accounting for super-Earth formation around many M stars. In an earlier paper, Boss and colleagues had already suggested that our Solar System formed in a region of high-mass star formation, so that the radiation from massive stars was able to photoevaporate the gas envelopes of the two outermost gas giant protoplanets Uranus and Neptune, leaving a rock/ice core and a much reduced atmosphere.

We’re left with the deduction that a preponderance of red dwarfs should be orbited by super-Earths rather than gas giants, and a mechanism that explains why gas giants do occur around those that emerged from less dense stellar nurseries. The paper is “Rapid Formation of Super-Earths around M Dwarf Stars,” available here.

Centauri Dreams‘ take: What about Gl 876, where a super-Earth is found inside the orbits of two outer gas giants? Boss acknowledges that the disk instability scenario developed in this paper cannot explain such a system. Gl 876, in his view, contains a super-Earth that developed “…by the same collisional accumulation process that led to the formation of the terrestrial planets in our Solar System.” So here’s his overview, creating for M stars the same three classes of planets found in our Solar System:

  • Inner terrestrial worlds formed by collision and accumulation of debris
  • Outer gas giants or rock/ice giants (super-Earths) formed by disk instability, their subsequent evolution depending upon the presence or absence of strong fluxes of UV radiation
  • One other thought: While the microlensing and radial velocity work on M stars is fruitful, the sample we have to work with remains quite small. And I can’t help wondering how much these early deductions are going to change as our datasets become more robust. Thus far, every step in the exoplanet hunt has produced yet another surprise, a process that is presumably not over.