At face value, a team of astronomers led by Andrew Howard and Geoffrey Marcy (University of California, Berkeley) has made an audacious claim. Based on observations of 166 nearby stars like the Sun in mass and brightness, the observers calculate that Earth-mass planets should be orbiting roughly a fourth of such stars. But they haven't found a single one of these putative Earths — it's an entirely statistical argument.
Here's the logic behind their conclusions, which appear in tomorrow's issue of Science. These observers have been toiling at Keck Observatory for five years to identify low-mass planets around 230 solar-type stars within about 80 light-years of us. They've been using a tried-and-true technique: noting wobbles in the stars' back-and-forth motion, using sensitive radial-velocity measurements, that betray the gravitational tug of a world in orbit around it. The bigger the wobble, the more massive the planet; the faster the wobble, the shorter the planet's orbital period.
The team's final tally shows 33 planets around 22 of the candidate G- and K-type stars, and about half of these orbit in less than 50 days (putting them within about 25 million miles of their host stars). Odds are good that all the really massive planets, Jupiters on up, have been identified, because they create the most obvious wobbles in their stars. Yet only a few of these were detected.
A higher percentage of the finds fall into the categories of "Neptunes" and "super-Earths", those at least 3 to 30 Earths in mass. But their weaker gravity translates to smaller stellar wobbles, and the team assumes that more than half of the planets in this mass range await discovery.
Anything smaller still, close to Earth in mass, would have escaped detection — the state of Doppler spectroscopy just isn't good enough to spot the anemic wobbles they induce in their stars' motion — and, in fact, the survey didn't turn up a single Earth-class planet. But the trend seems clear: for every "Jupiter" there are many more "Neptunes" and "super-Earths" and, by statistical extrapolation, even more "Earths." In fact, since the survey was limited to close-in (50-day) orbits, the authors argue that even more low-mass planets could be crowding around all those alien Suns if the search zone were extended out to 1 astronomical unit (Earth's orbital distance from the Sun).
Interestingly, past simulations of planet formation suggest just the opposite, predicting a deficit of planets having 5 to 30 Earth masses and orbital periods of less than 50 days.
So who's right? I'm no statistician, so don't ask me! A better bet would be to wait for the full results from NASA's Kepler mission, now in the midst of staring down 156,000 stars for evidence of planets that transit their stars. Kepler is able to detect Earth-size worlds, and Howard's team estimates that a couple hundred of them (up to twice Earth's diameter) should eventually turn up in the spacecraft's observations of stars like the Sun.