The National Science Foundation’s press release on the twin planetary discoveries recently announced by NASA can be found here. In it, Geoff Marcy suggests that while lower-mass planets are much harder to detect than the kind of gas giants that have so far dominated extra-solar planetary discovery, they may be much more common. And the implication from that is that Earth-size worlds may be ‘downright abundant.’

Re the Gliese 436 planet, the primary instruments were the two Keck telescopes at Mauna Kea, which monitored 950 nearby stars, 150 of which were tiny M-class red dwarfs:

The effort paid off in July 2003, when the astronomers noticed a periodic wobble in Gliese 436, an M dwarf star that lies about 33 light-years from Earth in the direction of the constellation of Leo. Another 12 months of data-taking confirmed the result: Gliese 436 has a Neptune-sized planet of at least 21 Earth masses that goes whipping around in its circular orbit once every 2.64 days. That corresponds to an orbital radius of roughly 4.5 million kilometers, or about 3 percent of Earth’s distance from the sun.

From our terrestrial perspective, this does make for a rather bizarre kind of solar system. Even Mercury, the closest planet to our sun, has an orbital period of 88 days and an orbital radius of 58 million kilometers—more than 12 times further out. And our own Neptune has an orbital period of 165 years and an orbital radius of 4.5 billion kilometers: a thousand times further out. Still, for reasons that no one really understands, such close-in giant planets seem to be the rule among the exo-solar systems discovered so far.

Still, we know so little that I’m uncomfortable with any extrapolations. Surely the reason we’re seeing close-in gas giants around most stars where we’ve found planets is that these are the most susceptible to the ‘wobble’ analysis by which we find planets in the first place. Just how common such systems are is unknown, and will remain so until we get more instrumentation into space.