Astronomers have snapped what they say is the best photographic evidence yet of planets orbiting other stars. Two new planetary systems have been imaged in the Milky Way; one offers the first glimpse of a system with multiple planets.
Other possible planets have been imaged near stars. But the new pictures are the first to capture the subtle crawl of planets around their host stars, confirming that they are indeed in orbit.
"It's great to see the quest for direct imaging of extrasolar planets finally bearing fruit," says Ray Jayawardhana of the University of Toronto, who was not associated with the two new studies.
Direct imaging allows astronomers to detect planets orbiting at much greater distances from their stars than the techniques most commonly used today.
Since the planets are far from the glare of their host stars, their light can be studied to reveal planetary properties, such as temperature and composition.
Two teams of astronomers made the discoveries, which may challenge the prevailing model of how planets form.
Christian Marois of the Herzberg Institute of Astrophysics in Victoria, Canada, and colleagues found a system with three planets in an ongoing survey of 80 young stars surrounded by relatively large amounts of dust.
This near-infrared composite image shows the nearby star HR 8799 (multi-coloured blob) and its three planets (red dots at upper left, upper right and just below the star). The planets are 7 to 10 times as massive as Jupiter (Image: National Research Council Canada)
Debris discThe planetary trio orbits the star HR 8799, which sits 130 light years away in the constellation Pegasus. The most distant planet in the group is roughly 7 times the mass of Jupiter and sits 68 astronomical units away from its host star (1 AU is the distance between the Earth and Sun).
Two larger planets, each estimated to weigh 10 Jupiters, orbit closer in - at 24 and 38 AU away from the star. A disc of dusty debris, similar to the solar system's icy Kuiper beltMovie Camera, surrounds the entire system.
Like other photographed planetary candidates, HR 8799's three planets weigh close to 13 Jupiters, a mass thought to separate planets from failed stars called brown dwarfs.
But the trio are most likely planets, says Marois. While stars orbit each other at random orientations, HR 8799's companions all seem to orbit in a plane, suggesting they formed in a disc like planets. Astronomers have also never found a system with three orbiting stars, he says.
The planets, which formed some 60 million years ago, are still glowing with heat from their contraction. Their march across the sky with the star, as well as their counter-clockwise orbit around it, were measured in near-infrared images taken with the Keck and Gemini North telescopes in Hawaii.
Bright glintThe team used a technique called adaptive optics to produce clearer images by removing distortions from the Earth's atmosphere.
In a separate study, another planet was found orbiting the star Fomalhaut, which sits some 25 light years from Earth and is surrounded by a belt of cold dust.
Astronomers predicted in 2005 that a planet might orbit Fomalhaut, after Hubble Space Telescope images showed the inner edge of the star's dust belt was unexpectedly sharp and not centred on the star. Astronomers suspected a planet might have carved this shape.
Comparing Hubble observations from 2004 and 2006, Paul Kalas of the University of California, Berkeley, and colleagues found the bright glint of a planet. Called Fomalhaut b, it sits 119 AU from its host star and weighs no more than 3 Jupiter masses, based on its gravitational effect on the dust belt.
A ring of dust and debris surrounding the star Fomalhaut (whose light is blocked out) appears here in an optical image taken with the Hubble Space Telescope in 2006. A planet (inset) orbits the star about three times as far from its star as Pluto does from the Sun - a separation larger than that of any other confirmed exoplanet (Image: Paul Kalas/UC Berkeley/NASA/ESA)
Record distancesThe team is still struggling to explain why the planet is unusually bright in one of the two wavelengths of optical light used to study the planet. Hot gas might enshroud the planet, or a ring of gas and dust that extends over 1 million km might girdle it. A ring of debris probably encircled Jupiter when it was 200 million years old - the age of Fomalhaut b, says Kalas.
The team says follow-up observations with Hubble Space Telescope's Space Telescope Imaging Spectrograph, which is set to be repaired on the last shuttle mission to the probe, could help explain the excess brightness.
Hundreds of extrasolar planets have been found by measuring the velocity changes in the star caused by the tug of its companion or by studying the dips in light levels when a planet passes in front of its host star. But these techniques have only turned up planets within 6 AU of their host stars.
The newly discovered planets - which all lie at least four times that distance from their host stars - could challenge the leading model of how planets form, says Jayawardhana.
Duelling modelsAstronomers suspect gas giants coalesce through 'core accretion', in which dust particles slowly stick together to become rocky chunks called planetesimals. These chunks collide to build a massive solid core before attracting an outer layer of gas, a process that takes millions of years.
But far from a star, matter in the disc thins out and takes longer to orbit. This might mean that distant, nascent planets do not have enough time to form a core before a star's radiation blasts away the surrounding gas.
"My hunch is that core accretion is going to find it challenging to make such giant planets so far away for their parent stars," Jayawardhana told New Scientist.
Exoplanet discoveries have challenged the idea of core accretion in the pastMovie Camera. But Jayawardhana says the newly discovered three-planet system might be "the closest to poster children as you can find for seriously considering disc instability", an alterative theory for how some gas giant planets could form.
In this theory, gravitational instabilities in a denser disc of debris can cause knots of matter to collapse rapidly, forming planets in thousands, rather than millions, of years.
Journal reference: Sciencexpress (doi: 10.1126/science1167569 and 10.1126/science.1166585)
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