The Hubble Space Telescope has uncovered a crop of 16 possible planets circling stars near the bustling centre of the Milky Way. Five are whipping around their stars in less than a day, giving them the shortest “year” on record.
Unlike the vast majority of extrasolar planets found to date, the new ones are also very distant, lying 26,000 light years away – well beyond our own galactic suburb. That suggests they are common throughout the Milky Way, which probably contains billions of planets.
“Now we are more confident in extrapolating perhaps through the entire galaxy,” says Kailash Sahu at the Space Telescope Science Institute in Baltimore, Maryland, US.
Sahu’s team discovered the new planets when they used Hubble to study a dense star field in the constellation Sagittarius. Computer analysis of the observations suggested that 16 of the stars periodically dimmed by up to 10% due to orbiting planets crossing their faces and blocking some starlight. Watch an animation of a planet circling its star.
The observations suggest the planets are roughly as wide as Jupiter. They circle stars that are typically fainter and smaller than the Sun.
The astronomers were able to estimate the masses of just two of the planets using the Very Large Telescope in Chile. One was less than 3.8 times as massive as Jupiter, the other roughly 10 times as massive as Jupiter.
Five of the worlds have tiny, tight orbits with periods of just 10 to 23 hours. Sahu says that viewed from the planets, their parent stars could appear some 60 times wider than the Sun looks from Earth.
All the planets found before now, mainly circling nearby Sun-like stars, have periods longer than 1.2 days. “Locally, we have found a surplus of planets in orbits with periods of less than three or four days, but suddenly at one day, they stopped – there was nothing below one day,” says Sahu.
Possibly, the new planets only survive so close to their stars because the stars are fairly dim and not hot enough to boil the planets away. “If evaporation destroys planets near bright stars, then for a low-mass star, they should be able to come a bit closer,” says Sahu.
Alternatively, the finding could be telling astronomers something about how giant planets migrate. These planets are thought to form in the cool outer region of a disc around a young star, then spiral inwards due to drag from the disc, stopping where the disc cuts off close to the star.
The new ultra-short-period planets around dim stars might suggest that the discs around small, faint stars cut off very close to the star. This would allow the disc to drag any planets in closer than ever.
Sahu stresses that the masses of most of the new potential planets have yet to be measured directly. A few of them might turn out to be small, dim stars instead of planets, but astronomers will need to wait for the next generation of space telescopes to find out.
Journal reference: Nature (vol 443, p 534)
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11:17 21 August 2008
