Smaller and smaller planets keep coming into view. A prime goal, of course, is to find something around the size of the Earth, implying as it would the existence of a world that might be like ours in other ways. My suspicion is that one day soon a transit study is going to come up with an exoplanet that’s closer to the size of Mars (definitely possible with today’s technologies), and we’ll skip right past the ‘Earth twin’ point before finding a planet that really is close to the same diameter.

But so far we’re still looking at worlds larger than Earth, like the tongue-twisting MOA-2007-BLG-192Lb, now thought to be the lowest mass planet ever found around another star. Announced today at the American Astronomical Society’s meeting in St. Louis, the new planet orbits a brown dwarf. At six percent of the mass of the Sun (and thus unable to sustain nuclear reactions in its core), the host is the lowest mass star to have a companion with a planetary mass ratio. But the fudge factor in the data does allow for the possibility of it being an extremely low mass hydrogen-burning star rather than a brown dwarf.

Smallest planet yet found

The news conference making these announcements is just winding down, a major point being that before this discovery, no planets had been found around stars less than twenty percent as massive as the Sun. We’re finding out that low mass stars can indeed host planets in the Earth-mass range, making nearby red dwarfs more and more interesting as systems with astrobiological possibilities. And with hundreds of brown dwarfs within 100 light years of Earth (fifteen percent of them in binary systems), we now must factor in the kind of planets we can expect to find around them.

Image: Artist’s conception of the newly discovered planet MOA-2007-BLG-192Lb orbiting a brown dwarf “star” with a mass of only 6% of that of the Sun. Theory suggests that the 3-earth-mass planet is made primarily of rock and ice. Observational and theoretical studies of brown dwarfs reveal that they have a magenta color due to absorption by elements such as sodium and potassium in their atmospheres. Credit: David Bennett.

This particular world has an orbital radius similar to that of Venus, but in a system dominated by such a faint central star, temperatures at the top of the atmosphere would be frigid. Even so, the possibility is strong that the planet has a thick atmosphere, one that would allow warmer temperatures on the surface, and there is some speculation that interior heating by radioactive decay could keep that surface warmer still. We may be dealing with a vast, deep ocean under all that atmosphere. But let me quote the paper on this:

…it is possible that MOA-2007-BLG-192Lb could have a habitable surface temperature itself, despite the fact that its host star or brown dwarf provides extremely feeble radiative heating. Stevenson (1999) has speculated that even a free ?oating Earth-mass planet could have a surface temperature that would allow liquid water even though the heating from internal radioactive decays provides a factor of ? 104 times less energy than the Earth receives from the Sun. The key point of Stevenson’s argument was that such a free ?oating planet might retain a molecular Hydrogen atmosphere that could provide very strong insulation that would allow the surface temperature to remain above the melting point of water ice. If it was possible to detect nearby analogs to MOA-2007-BLG-192Lb, it would be worthwhile to attempt to study their spectra to see if they do have H2 atmospheres that might allow warm surface temperatures.

Congratulations to two microlensing teams — the Microlensing Observations in Astrophysics (MOA) and Optical Gravitational Lensing Experiment (OGLE) collaborations — which used lensing to detect the planet. In this method, light from a background star is magnified by an intervening star (in this case, the planetary host), with the planet being spotted by additional perturbations to that light. I’m now looking at a press release on this work and noting a statement by David Bennett (University of Notre Dame), who led the team: “I’ll hazard a prediction that the first extra-solar Earth-mass planet will be found by microlensing. But we’ll have to be very quick to beat the radial velocity programs and NASA’s Kepler mission, which will be launched in early 2009.”

Yes, better hurry, for each planet detection method is improving rapidly, but my money is still on the transit method to make that first Earth-mass discovery. The paper is Bennett et al. (46 co-authors!), “A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192,” accepted by the Astrophysical Journal. I’ll have the arXiv link up as soon as it becomes available.