For nearly 15 years planet hunters have scanned the sun's cosmic backyard for other solar systems. In the process, they've bagged more than 200 planets circling other stars. Now, a team using the Hubble Space Telescope has extended the hunt to the heart of the galaxy and found evidence for as many as 16 more.
The "candidates" exist among a population of stars up to 26,000 light-years away, in the direction of the constellation Sagittarius. They include Jupiter-class planets that orbit their suns in less than 24 hours, the astronomers announced Wednesday.
One candidate, a scant 740,000 miles from its dwarf star, zips around in as few as 10 hours, the team estimates. The star's relatively low temperature and the planet's large mass help explain why the planet – a searing 3,000 degrees F. on its day side – hasn't evaporated away, the team says.
Discovering the planets with very short orbits "was a big surprise," says Kailash Sahu, an astronomer with the Space Telescope Science Institute in Baltimore, who led the team.
The results are encouraging for astronomers looking to a new generation of space telescopes to find more planets beyond the solar system humans call home. Dr. Sahu's team in effect has validated a method that is expected to become a mainstay for space-based telescopes that can't directly image the objects.
The Hubble team looked long and hard at a single patch of sky containing some 180,000 stars. Then it looked for a faint, tell-tale dimming as a planet candidate passes between the telescope and a star.
Ground-based telescopes have used this approach to detect 14 transiting exoplanets so far – including four in the past month. But the terrestrial efforts require astronomers to observe one star at a time, with targets usually selected after another technique spots evidence of a planet. The Hubble team, by contrast, gathered all its candidates at once by staring at a region of space over seven days. Little wonder it carries the acronym SWEEPS.
The French are getting set to exploit this mass-produced approach when they launch their COROT space telescope at the end of the year. The telescope is designed to observe star quakes and hunt for transiting planets.
"This is terribly exciting," says Sara Seager, a scientist at the Carnegie Institution of Washington who models exoplanets' properties. Assuming most of the Hubble candidates are confirmed as planets, "they will have doubled the number of known transiting planets" in one observation. "This shows that it's relatively easy to discover transiting planets."
The Hubble team's objects will remain "candidates" until planet status can be confirmed using another planet-hunting technique that detects the slight gravitational tug a planet imparts to its host star. Either technique alone is suggestive, notes Alan Boss, another Carnegie astronomer. The tug approach gives a lower limit to an object's mass, which tends to be close to its actual mass. The transit approach also helps nail down the mass, as well as the object's radius. The mass and the radius yield an estimate of the object's density – "and now you can do some real planetary science," he says.
By knowing a planet's bulk density, astronomers get a rough idea of whether the object is mostly gas, rock, or ice. And by looking for changes in the star's chemical fingerprints with and without the planet in the line of sight, astronomers can begin to tease out information about the planet's atmosphere.
The team says at least seven of the 16 objects are likely to be planets. Five are the short-orbit speedsters. And the team has used the tug, or radial velocity, approach to determine that the masses of two of the objects are too low to be anything other than planets.
The group's observations, which were conducted 2004, set records for the largest number of candidates captured in one field of view; the hottest, speediest planet yet discovered; and the region of space explored, Dr. Boss says. The findings were announced Wednesday at a NASA briefing and published Thursday in the journal Nature.
The discoveries in the sun's neighborhood has left many astronomers wondering how representative our solar system may be. Now, with snapshots of a distant galactic environment, plus data from closer to home, the Hubble team estimates that the galaxy may hold some 6 billion Jupiter-class planets.
The Hubble team's results "are not surprising, but they are reassuring," Boss says. "Planets are everywhere."
Still, the question remains: How many solar systems with a general planetary line-up like our own does the Milky Way hold? That, he adds, will continue to drive the hunt.