SWEEPS-10 orbits its parent star from a distance of only 740,000 miles, so close that one year on the planet happens every 10 hours. The exoplanet belongs to a new class of zippy exoplanets called ultra-short-period planets (USPPs), which have orbits of less than a day.

This story was updated at 1:45 p.m.EDT.



A recently spied planet orbits so close to its star that a new year comes every10 hours.

CalledSWEEPS-10, the planet [image]belongs to a newfound class of zippy exoplanets called ultra-short-period planets (USPPs) that have orbits of less than a day.

The Hubble Space Telescoperecently spotted five USPPs, all about the size ofJupiter, in a crowded star field [image]near the galacticbulge of our Milky Way galaxyas part of an exoplanet survey called the SagittariusWindow Eclipsing Extrasolar Planet Search (SWEEPS). Atotal of 16 planet candidates were found, all with relatively short orbitalperiods.

"Theseare the farthest planets detected so far around some of the faintest stars,"study leader Kailash Sahuof the Space Telescope Science Institute (STScI) inMaryland told reporters at a NASA press conference.

Extrapolatedto the entire Galaxy, the Hubble results suggest the Milky Way contains atleast 6 billion Jupiter-sized planets, researchers say.

The findingsare detailed in the Oct. 5 issue of the journal Nature.

The newplanets were discovered using the transit technique, in which researcherscalculate a planet's mass and size based on the periodic dimming of starlightthe planet creates when it passes in front of its parent star.

Previously,the shortest known orbital period for a planet was 1.2 days and belonged to a"hotJupiter," a giant gas planet that also orbited close to its star. Inour solar system, Mercury is theclosest planet to our Sun and has anorbital period of 88 days. Earth,located farther away, takes about 365 days to make one full orbit.

Unlike withhot Jupiters, the stars the USPPsorbit are small. The star that SWEEPS-10 orbits has less than half the mass ofour Sun. The stars' small sizes arelikely what allow the planets to orbit so close, the researchers say. Planets orbiting larger stars at suchclose ranges would be destroyed.

"Ifsun-like stars have this kind of planet close to the star, it would simplyevaporate the planet," Sahu said.

SWEEPS alsoconfirmed something that astronomers have long suspected: that stars rich inheavy elements like carbon and iron are more likely to have planets.

"Wewanted to now if this holds halfway around the galaxy," said study teammember Mario Livio, also of STScI."We found that indeed, it does hold. If you find a star rich in heavyelements, there's a high probability that there will be a planet around it."

Alan Boss,a planet expert at the Carnegie Institution of Washington who was not involvedin the study, said the new findings underscore the fact that the three types ofplanets found in our solar system--rocky planets, ice giants and gas giants--alsoappear to be abundant in the universe at large.

"We knowthat all three examples are out there, and that gives us great hope for thefuture of the field," Boss said.