Of other earths, other wonders
Astronomers at the $600-million satellite observatory named Kepler are pitching their hopes on a list of stars that they consider are their brightest bets for harbouring planets, some of which could turn out to be earth-like worlds. Kepler will, over the next few years, be able to detect planets in the Goldilocks zone, writes Dennis Overbye
In a building at NASA’s Ames Research Centre here, computers are sifting and resifting the light from 1,56,000 stars, seeking to find in the flickering of distant suns the first hints that humanity is not alone in the universe.
The stars are being monitored by a $600 million satellite observatory named Kepler, whose job is to conduct a Gallup poll of worlds in the cosmos. Kepler’s astronomers have recently identified 1,235 possible planets orbiting other stars, potentially tripling the number of known planets. They have also unveiled a closely kept list of 400 stars that are their brightest and best bets for harbouring planets, some of which could turn out to be the smallest and most earth-like worlds discovered to date.
Over the next few years, Kepler will be able to detect planets in the ‘Goldilocks’ zones, where it is neither too hot nor too cold for liquid water. “What we want is to find life,” said Geoffrey Marcy, an astronomer at the University of California, Berkeley, and part of the Kepler team.
William Borucki, 72, the lead scientist, who has spent the last 20 years getting Kepler off the ground, said, “I’ve argued that Kepler is more important than the Hubble Space Telescope. We provide the data mankind needs to move out into space.”
Where should next mission head to?
A roving robot laboratory named Curiosity will depart for Mars on a $2.5 billion mission this fall. Astronomers argue whether the next such mission should go to Jupiter’s moon Europa, Saturn’s moon Titan or another of Saturn’s moons, Enceladus.
Right now, humans cannot even summon the money or political will to get back to the moon, let alone set sail for another star. It would take 300,000 years for Voyager 1, now on the way out of the solar system at 39,000 mph, to travel the 20 light-years, or 120 trillion miles, to Gliese 581, one of the nearest planetary systems; Kepler’s planets are from 500 to 3,000 light-years away.
NASA and other organisations, like the Planetary Society, have experimented with devices like solar sails, in which a craft is pushed by sunlight or a powerful laser, and ion drives, in which high-energy particles do the propelling. It was only in 1995 that a team of Swiss astronomers led by Michel Mayor of the Geneva Observatory discovered the first planet of another sun-like star using what is now known as the “wobble” method.
A planet gives its star a little gravitational tug as it goes around, causing the star to wobble a little as both star and planet circle the same centre of gravity. They detected a wobble in the motion of the star 51 Pegasi as an object about half the mass of Jupiter whipped around it every four days.
Over the next decade, Mayor’s group and another team led by Marcy and R Paul Butler of the Carnegie Institution in Washington leapfrogged each other in finding exoplanets.
Astronomers have recorded direct images of four planets swirling around a star known as HR 8799, 130 light-years from earth in the constellation Pegasus, and another circling Fomalhaut, only 25 light-years from Earth. There are over 500 planets listed on the Jet Propulsion Laboratory’s PlanetQuest website, none habitable.
Among them are the Styrofoam planet, an early trophy of Kepler’s, a planet half as large as Jupiter. Another is a planet composed almost entirely of superheated water and sometimes called the Steam World; it is known as Gliese 1214b, about 40 light-years from here in the constellation Ophiuchus.
Discovery of Gliese 581g
Last year, a team of American astronomers announced the discovery of a Goldilocks planet orbiting a dim red dwarf star at just the right distance to harbour water on its surface, making it a potential site for life. Gliese 581g is part of the Gliese 581 system 20 light-years from here.
In 1984, Borucki suggested that precise measurements of light could be used to look for planets. The idea is that a planet passing in front of its star would block a little of its light – very little. In the case of the earth, the dip would amount to 84 parts per million in the sun’s light – less than a hundredth of a per cent.
After NASA scientists turned down a proposal by Borucki to build a satellite to do such measurements twice before, and giving them half a million dollars to spend on lab work, the Kepler mission finally got the nod from NASA in 2001, but with a twist. The Ames Centre wound up handing over management of the mission, at least until the launch, to the Jet Propulsion Laboratory in Pasadena, Calif. Kepler was launched from Cape Canaveral into an orbit around the sun on March 6, 2009.
Its gaze is fixed on a patch of sky about 20 full moons across near the Northern Cross, in the constellations Cygnus and Lyra, containing 4.5 million stars. That is the neighbourhood for Kepler’s cosmic census. The job is to measure the brightness of 156,000 of those stars every half-hour, looking for the repeated dips caused by planet crossings, or “transits.”
The more times a planet crosses its star, the more easily it is picked up and tagged by computers analysing Kepler’s data. Kepler’s first hits were indeed of planets that orbited their suns in a few days in close orbits that would produce even-cleaner temperatures. The earth, of course, takes a year to go around the sun, so it would take two or three years for its analogue orbiting some star in Cygnus to show up in the Kepler data. “We will find earth-size planets in habitable zones,” Marcy stated flatly last month.
So, what’s the hitch?
There is a hitch to confirming those planets, however. Such planets would not exert enough of a gravitational tug on their suns to be detectable by the “wobble” method, the main way their masses can be measured. Instead of confirming such planets, Kepler astronomers talk about “validating” them by using high-powered telescopes to make sure that there is only one star there and not a pair of eclipsing stars or some other phenomenon that could mimic a planet’s shadow.
Natalie Batalha of San Jose State University, the deputy science team leader for Kepler, said it could be that they will wind up with, say, 100 planets they are 80 per cent sure of, which could translate to 80 planets – useful for a census, not so helpful if you’re looking for a place to live.
The road map to new worlds, Batalha explained, goes like this: First, Kepler figures out how abundant Earths are and how far you have to go out into the universe to find one. That information is needed to design the next step – a mission that would search the sky for earth-like planets close enough to study. But at 500 to 3,000 light-years away, Kepler’s planets are too far for intense direct scrutiny.
One idea for such a mission is a “starshade” that would float in front of a telescope in space and cancel out the bright light from a star, allowing its much dimmer planets to stand out. Indeed, some astronomers have proposed building such a starshade for the James Webb Space Telescope, Hubble’s successor, scheduled to be launched by NASA later this decade. “It could potentially not only image an earth-like planet, but provide some information about its atmosphere and surface,” said David Spergel, an astrophysicist from Princeton.
New York Times News Service