Super-Earths point to crowded universe
— -- So, Sir Isaac Newton, it turns out, was no slouch: Amid his accomplishments in math, physics and optics, he prophesied the glut of planets discovered in the last two decades circling nearby stars. The 1713 edition of the venerable astronomer's Principia Mathematica describes our own solar system — "this most beautiful system of the sun, planets, and comets" — and speculates, "the fixed stars are the centers of other like systems."
The story of these "extrasolar" planets gained its latest chapter last week, at a Paris colloquium for France's Centre National D'Etudes Spatiales space telescope, COROT. The telescope searches a swath of nearby stars for signs of dimming caused by planets passing in front of them, so-called "transit" planets, and one reported at the colloquium, COROT-Exo-7b, was among the smallest yet detected, less than twice the size of the Earth, but much hotter.
"For the first time, we have unambiguously detected a planet that is 'rocky' in the same sense as our own Earth," said COROT's Malcolm Fridlund, in a European Space Agency statement. "This discovery is a very important step on the road to understanding the formation and evolution of our planet."
Almost 340 planets have been detected orbiting nearby stars, according to the catalogue maintained by Paris Observatory astronomer Jean Schneider. Most discovered since 1995 are Jupiter-sized, or larger, worlds, many of them "roaster" planets that nearly graze their star.
The last few years have brought reports of "Super-Earths," planets like COROT-Exo-7b that only outweigh our planet by a bit.
"Finding Super-Earths, roughly a dozen of them, has been profound, a major leap forward," says theorist Alan Boss of the Carnegie Institute of Washington, author of The Crowded Universe: The Search for Living Planets, released today. Judged by the rate of recent discoveries, about 30% of nearby stars likely possess planets in Earth's weight class, he says.
COROT's report clears the way for the planned March 5 launch of NASA's $591 million Kepler mission, a larger transit-spotting telescope that should be able to spot Earth-sized planets in the "habitable zone" around stars, where water neither boils away or freezes away.
"The whole point of Kepler is to find lots of Earths," says Boss, the mission's official theorist. "We hope to see even more Super-Earths."
The smallest transit planets uncovered by Kepler will be too small for confirmation by the "radial velocity" technique responsible for finding most of the worlds spotted since 1995. The technique records gravitational wobbles in stars triggered by companion planets.
"Once we find those Earths, there will be a decade or two of waiting for measurements of atmospheric methane and oxygen before we can say anything about life there," Boss adds. NASA's plans for "Terrestrial Planet Finder" missions which could make these measurements have receded due largely to the agency's budget woes, he adds. "Kepler has become NASA's terrestrial planet finder."
Over the next three years, assuming a successful launch, the mission will identify steadily smaller planets more distant from their host stars. About 10% of solar systems are thought to orbit edge-on to Earth, just by chance, and it will be planets in those alignments that Kepler will detect through transits. The feat requires the eyeballing of about 100,000 stars lying in the direction of the constellation, Cygnus, the Swan.
In the meantime, planetary discoveries of other sorts keep on coming. Spitzer space telescope astronomers recently reported HD 80606b, located 190 light-years away in the constellation Ursa Major (one light year is about 5.9 trillion miles), which follows an elongated orbit passing so close to its star that its atmosphere's temperature rises from 980 degrees Fahrenheit to 2,240 degrees in about six hours time, a shocker of a summer.
"If I have seen further it is only by standing on the shoulders of Giants, wrote Newton, who designed the first simple mirror-using telescope in 1670. From transit to radial velocity detection, his tricks still play a part in the very latest discoveries, one he predicted three centuries ago.