Feeling special today? Get over it.
A new study by astronomers at University of California at Berkeley shows when it comes to planets, smaller means more, a finding that has strong implications for the prospects of relatively puny planets like Earth appearing in other solar systems.
So far, the smallest extrasolar planets found to date are about two to three times Earth's mass, but a random survey of 166 nearby stars shows the extended family of planets is fairly robust. Twenty-two of 166 nearby stars, selected at random, had a total of 33 planets, data teased out by breaking down the starlight to look for tiny aberrations caused by the gravitational tugging of the orbiting brood.
The study, which took five years, only looked for planets that orbited rather close to their parent stars (unless the planet is very large the signal from its gravitational hug is too slight to detect with today's technology) so this batch won't produce good candidates for worlds with liquid surface water that might be suited for life.
Still, planet-hunter Geoff Marcy says it's the first statistical estimate of just how many pale, blue dots may be out there.
"We really had no estimate at all of what fraction of stars might harbor Earth-like planets," Marcy told Discovery News. "We just had no way of knowing from theory or observations."
The study found one or two stars with planets about the size of Jupiter, six with a Neptune-mass world, and about 12 with super-Earths, roughly three to 10 times the mass of Earth. Although the planets orbit their parent stars at just .25 percent of the distance between Earth and the sun, the astronomers say there's no reason to believe the size distribution of planets would be different as you move farther away.
"We detected a steeply rising rate of planet occurrence as you lower the mass. Since we see this strong trend, it'd be surprising if it just turned over or shut off immediately," lead author Andrew Howard told Discovery News.
"What we've done, in effect, is take a census. We've shown that there are more and more planets are you get smaller," added co-author Marcy.
The astronomers extrapolate from their data that the proliferation of smaller planets continues down to worlds the size of Earth -- and presumably even smaller -- planets that are beyond the technical capabilities of the study.
"I think it's very speculative," Massachusetts Institute of Technology astronomer Brice-Olivier Demory told Discovery News. "It looks to me very, very, very difficult to extrapolate the low-mass (planet) population. I think it's too premature to draw any conclusion."
"We detected a steeply rising rate of planet occurrence as you lower the mass. Since we see this strong trend, it'd be surprising if it just turned over or shut off immediately," he said.
More information will be coming soon. A Geneva-based team of planet-hunters has made preliminary assessments that 30 percent to 50 percent of its surveyed stars have planets. And scientists using NASA's Kepler space telescope, which is on the hunt for Earth-sized worlds, has yet to weigh in .
"We've shown that planets become more common as you move to low mass, if that continues as you move outward, then there are potentially lots of Earth-like planets in habitable zones," Howard said.
The research appears in this week's issue of Science.