Maybe it’s time to put some new numbers into the Drake Equation. That’s the formula, developed by astronomer Frank Drake in 1961, that estimates the number of civilizations in the galaxy which are sufficiently advanced to have harnessed the electromagnetic spectrum—a fancy way of saying they have radio waves, TV and other components of technology that we could detect even from here.
Not only does the number of planets outside our own solar system continue to increase—it’s now up to 294 —but the range of stars that have planets and the range of sizes of the planets themselves also keep expanding. Today, for instance, astronomers are reporting the discovery of the smallest extrasolar planet yet (only three times more massive than Earth), orbiting a star only one-twentieth the mass of our Sun. That suggests that even stellar lightweights, which are relatively common, can have a retinue of earthlike planets.
“No planets have previously been found to orbit stars with masses less than about 20 percent that of the Sun,” said David Bennett of the University of Notre Dame, who led the study. “But this finding indicates that even the smallest stars can host planets.”
The planet popped up through gravitational microlensing. When one star passes directly in front of another as seen from Earth, its gravity acts like a giant magnifying glass, warping the surrounding space. The background star brightens. If a planet is orbiting the passing star, the starlight is warped, revealing the presence of the planet.
The passing star in this case has the unfortunate moniker MOA-2007-BLG-192L (MOA is the Microlensing Observations in Astrophysics, the New Zealand observatory that spied the star and its planet; 2007 is the year the microlensing event occurred; BLG means bulge; 192 means the 192nd microlensing observation by the MOA and L indicates the lens star rather than the background star). The planet is designated with a final letter: MOA-2007-BLG-192Lb. We’ll call it 192Lb for short.
It’s about 3,000 light years away. Its star is either a low-mass one that burns hydrogen, as our Sun does, or a brown dwarf, too lightweight to sustain nuclear fusion. It’s the lightest star ever found with a planetary entourage (of one, but still . . . ), and the planet is also the new record-holder for least-massive planet outside our solar system, at three times Earth’s mass.
If very low-mass stars have planets with a mass comparable to Earth, that offers many potential targets for the planned James Webb Space Telescope, which might be able to search for signs of life on them.
The star’s tiny size means it is very faint, and 192Lb is in a deep chill. But the astronomers say that although the top of the planet's atmosphere is probably colder than Pluto, it very likely has a massive atmosphere that would make lower altitudes warmer. It is even possible that radioactive decays in the planet’s interior, which also occurs inside Earth, would make the surface as warm as Earth’s.
A paper describing this result has been accepted for publication in the Astrophysical Journal, for the September 1 issue. You can find it, as well as very cool artist’s conceptions of the planetary system, here.