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Planet quest gets small

David A. Aguilar / CfA
This artwork shows the "super-Neptune" planet.

Normally blue in color, its

red hue is caused by the

illumination from the

nearby dwarf star.

A network of small telescopes has bagged its smallest prize yet - and that's great news for astronomers.

HATNet's discovery of an extrasolar world only slightly bigger than Neptune helps prepare the way for an even more capable planet-hunter that could find alien Earths.

To date, more than 300 planets have been found orbiting other stars. Most of them were detected by measuring the gravitational wobble that the planet induces in its parent star. That method works great for huge planets, but not as well for smaller worlds.

The Harvard-Smithsonian Center for Astrophysics' HATNet uses a different method: Six automated 4.3-inch telescopes in Hawaii and Arizona work together to watch for the ever-so-slight dimming of a star's light as a planet crosses in front of it.

By analyzing how the light pattern changes during the transit, astronomers can figure out how big the planet is. The wobble method can then be used to follow up on the detection and calculate the planet's mass. This interactive presentation explains the workings of the transit method as well as the wobble method.

A couple of years ago, HATNet found the most massive planet ever detected using the transit method. Its latest discovery, designated HAT-P-11b, is the smallest planet on its list. It is not, however, the smallest extrasolar planet known to orbit a normal star: That distinction belongs to a team of researchers who used a different method, called microlensing, to detect a planet just three times as massive as Earth.

HAT-P-11b is a "super-Neptune": It's 25 times as massive as Earth, and 4.7 times Earth's size. In comparison, the Neptune we all know and love is equal to 17 times Earth's mass and 3.8 times Earth's size. The planet orbits a dwarf star 120 light-years away in the constellation Cygnus, at a distance so close that it's baked to a temperature of 1,100 degrees Fahrenheit (600 degrees Celsius). One orbit takes just 4.88 days, according to a news release issued last week by the Center for Astrophysics.

The center reported that there are signs of a second planet in the same star system, but more gravitational-wobble readings are required to confirm the observation and nail down the second planet's properties.

Another "hot Neptune," known as GJ 436b, was found using the wobble method and then studied by a different team using the transit method. Now the scientists will be able to compare notes.

"Having two such objects to compare helps astronomers to test theories of planetary structure and formation," said Harvard astronomer Gaspar Bakos, who led the discovery team and has submitted a paper on the find to the Astrophysical Journal.

Hundreds of small worlds, including planets the size of Earth, might be found using the transit method once NASA's Kepler mission goes to work. The one-ton Kepler spacecraft is due for launch in March, and the HAT-P-11 star system should be in its field of view.

"We expect Kepler to measure the detailed properties of HAT-P-11 with the extraordinary precision possible only from space," said Harvard's Robert Noyes, another member of the discovery team.

Update for 7:20 p.m. ET Jan. 22: New Scientist quotes researchers as saying that the smallest known extrasolar planet orbiting a normal star may be even smaller than they originally thought. Their latest estimate suggests that the planet is only 1.4 times as massive as Earth. Even though the mass is Earthlike, the planet's environment probably isn't. "Because its host is a very dim red dwarf, the planet is likely to be frozen - even though it orbits at about the same distance as Venus from our sun," New Scientist reports.