Search for worlds like ours begins in islands

Earth-like planets are probably exceptionally common in our galaxy, and astronomers should look for them around the closest stars to ours, a prominent planet hunter suggests.

"The day may come, 50 or 100 years from now, when humans will send robotic spacecraft to Alpha Centauri and other nearby stars, so the nearest stars are quite important," says Geoff Marcy, an astronomer at the University of California at Berkeley whom Time magazine last year named one of the 25 most influential space scientists.

Key to the search is the Keck Observatory on Mauna Kea, says Marcy, who was on Hawaii island this month to mark Keck’s 20th anniversary.

"This is a fabulous time, obviously, in astronomy, the Keck Observatory playing the crucial role," Marcy said in a presentation at the Fairmont Orchid hotel in Wai­ko­loa. "There is an area, the search for planets around other stars — and especially answering an age-old question of how unique the earth itself is — that is being answered by the Keck Observatory."

So far the most concentrated hunt for so-called exoplanets (any planet orbiting around another star) has been focused on distant stars in a relatively small patch of sky near the constellations Cygnus and Lyra. That’s where the NASA’s Kepler space telescope, launched in March 2009, watches stars for signs of the slightest dimming in brightness.

"Kepler measures brightnesses of 150,000 stars, watching to see if any of them dim in a repeated, periodic way," says Marcy. "And of course a star would dim if a planet crosses in front of it, blocking a little of the starlight, dimming that star over and over and over again."

The phenomenon is no different from the transit of Venus across the sun last June.

"From space you can measure the brightnesses of stars to a part in 10,000, which is exactly the ratio of the cross-sectional area of the Earth to that of a sun-sized star," said Marcy. "So you should be able to see the dimming of stars caused by relatively tiny, Earth-like planets using this space-borne telescope."

So far, more than 2,000 such "candidate planets" have been spotted by Kepler.

If the type of star is known, the amount of dimming points to the size of the planet.

"Here’s the problem," Marcy says. "We don’t know whether this planet is rocky like Earth or whether it is made of gasses like Jupiter and Saturn and Neptune, or maybe water and gasses as well as rock. So we need more information than just the size of the planet. We need the mass of the planet, which will tell us the density."

That’s when the Keck steps in.

With a sensitive spectrograph called HIRES, which breaks out sunlight into its constituent wavelengths, the Keck can spot tiny shifts that indicate the star is moving toward us under the influence of the passing planet.

Those Doppler shifts are similar to the change in sound when a train whistle or car engine is approaching or moving away.

"The idea is to watch for the wobble of the star that tells you the mass of the planet as the planet yanks gravitationally on the star," he said. "We zoom in on dark spectral lines and measure tiny Doppler shifts of no more than one one-thousandth of one pixel. That corresponds to about 1 meter per second, literally human walking speed. We can tell as the spectral lines Doppler-shift back and forth whether the star is walking toward us or away from us — at 1,000 light-years away."

Anything in the range of Earth’s density, 5.5 grams per cubic centimeter, is almost certainly a rocky planet versus a gas giant, he said.

Kepler 10b, 560 light-years away, was the first definitive rocky planet ever found around another star, with a density of 8.8, roughly that of an iron dumbell.

Its discovery was announced in January 2011. Kepler 10b is about 40 percent larger than Earth, but it’s quite inhospitable.

"It’s so close to the star that it would be blow-torched on one side, frigid, cold, dark on the other side," Marcy said.

Astronomers are still looking for an Earth-size planet in the Goldilocks zone, an orbit not too near nor far from its sun, where water could exist in liquid form.

While they represent only a 10-by-10-degree section of the sky, the Kepler observations have confirmed that exoplanets exist in abundance and that Earth-like planets are also probably common.

"Of the 200 billion stars in the Milky Way, we wonder what fraction of those stars might harbor Earth-like planets, and we still don’t know the answer," said Marcy. "The Keck Observatory should be surveying the nearest 50 to 100 stars with Doppler measurements to try to find those Earth-like planets."

A proposed highly sensitive spectrometer, called the Stable High-Resolution Echelle for Keck, or SHREK, could help answer that question, he said.

He pointed to a recent calculation made by a Harvard graduate student named Courtney Dressing: "If our Milky Way galaxy were shrunk to the size of the United States, then the nearest star that has an Earth-like planet would be across the Golden Gate Bridge."

Marcy said he is also intrigued by the fact that most planets found by Kepler are between the size of Earth and four times that of Earth — that is, the size of Neptune.

"What is going on?" he said. "What are those planets? In our own solar system, there are no planets at all between the size of the earth and that of Neptune. There is nothing in between. The dominant planet being found by the Kepler telescope is a size domain unrepresented by our own solar system. We don’t know what these are made of. We don’t know how they form. And they are literally mysteries."