Closing in on Extrasolar Earths
This artist's impression shows the newly discovered trio of super-Earths orbiting a sun-like star, HD 40307.
Credit: ESO.

Little more than a decade ago, astronomers Michel Mayor and Didier Queloz announced the discovery of a planet in orbit about 51 Pegasi. It rushes around its sun in just over four days, seared to a temperature of 1,000 degrees Celsius (about 1,800 degrees Fahrenheit).

Today, we call this sort of planet a "hot Jupiter." This was the first planet found orbiting a main sequence star — a star similar to our sun.

Earlier, the irregular beat of a pulsar revealed the cindered remains of planets orbiting the corpse of a dead star. Most likely, they formed after the supernova death of their parent star, which indicates that planet formation is a likely outcome from a disk of material. These pulsar planets are not good places to live. But discovering 51 Pegasi around a more ordinary star kicked off a great planetary gold rush.

Almost weekly, there's an announcement of yet another extrasolar planet around a neighborhood star. This week, it's a triple system of super-Earths discovered at the European Southern Observatory at La Silla using the HARPS (High Accuracy Radial velocity Planet Searcher) instrument.

The planets, having 4.2, 6.7, and 9.4 times the mass of the Earth, orbit the star with periods of 4.3, 9.6, and 20.4 days, respectively. These short periods place the trio of new planets well within the orbit of Mercury around the sun. So, again, these are hot planets, well inside the habitable zone of their star. This trio, and others, were announced this week at the international conference, "Extra-solar Super-Earths," and demonstrate that small planets are likely common.

"Clearly these planets are only the tip of the iceberg," says Michel Mayor, team leader for this discovery. "The analysis of all the stars studied with HARPS shows that about one third of all solar-like stars have either super-Earth or Neptune-like planets with orbital periods shorter than 50 days."

These planets are like the proverbial gold nuggets sought by the 49ers here in California. They mark the trail to the mother lode, where the ultimate goal is to find another Earth.

Only a handful of the 300-plus planets known to exist are categorized as super-Earths. The ground-based radial velocity method preferentially reveals Jupiter-sized planets. They induce greater spectral shifts in their stars. Tiny terrestrial planets the size of Earth hardly disturb their parent stars, and their radial velocity signatures are lost in the noise.

Super-Earths are beginning to be found with high-precision instruments like HARPS. Hints of small planets have also been reported by the COROT (COnvection ROtation and planetary Transits) Mission and by the gravitational lensing experiments underway.

But the holy grail — an Earth-size planet in the habitable zone of its star — awaits NASA's Kepler Mission.

The Kepler spacecraft launches in February 2009, 400 years after Johannes Kepler published his first two laws of planetary motion in Astronomia Nova, the "New Astronomy," which first described planets orbiting on ellipses and at varying speeds around the sun. Ten years later, he published his third law, which relates the period of the planet to its mean distance from the sun. Kepler used his discoveries to predict solar transits of Mercury and Venus, but did not survive to observe them.

Soon, NASA scientists will seek transits to discover Earth-size planets about distant stars. And, they named the mission in honor of Johannes Kepler.

The Kepler Mission is especially designed to discover small planets around sun-like stars by observing transits. The Kepler Mission will observe more than 100,000 stars for at least 3.5 years, seeking evidence of other Earths. Lots of hot, close-in planets will be discovered in the first months of the mission.

Finding Earth-size planets in Earth-like orbits requires patience, because observations must be repeated to confirm discoveries. If an alien astronomer in a distant solar system were looking for us, Earthly transits would be seen once per year, and that good astronomer would require at least three transit observations to announce a discovery. The same is true for the Kepler scientists. Kepler's scientific prospectors should be announcing discoveries of Earth-size planets in habitable zones by 2011-2012.

In my opinion, that's a short wait to answer the question, "Are there other planets like Earth orbiting distant suns?" We're approaching the mother lode of distant solar systems, and the Kepler Mission promises a major advance toward understanding whether we are alone in this vast universe.