photo: The Hubble Space Telescope captured this image of a dusty disk around Fomalhaut. The inset image shows the position and motion of Fomalhaut b from 2004 to 2006. The object has the brightness one would expect of a planet similar in mass to Jupiter, given the system's age. Fomalhaut b's gravity has clearly not destroyed the disk, which also argues for planetary status. The star itself has been blotted out by an occulting disk. Click on the image for a larger view.
Paul Kalas (UC, Berkeley) / STScI / NASA
For years, astronomers have been racing one another to take the first picture of a planet orbiting another star. Over the past few years, several teams have claimed to have directly imaged an extrasolar planet. But in each case, there were lingering questions about the nature of the purported planet. The objects seem unusually massive for planets, and each orbits much farther from its host star than Pluto orbits the Sun. Many astronomers argue that these objects are more accurately described as failed stars (known as brown dwarfs) rather than true planets, because they probably formed from collapsing gas clouds, like stars.
Today, two teams of astronomers announced new exoplanet images, and in each case, I think they have the real deal. Only time and future observations will let us know for certain, but these objects have the look and feel of bona fide planets. One group found a planet orbiting Fomalhaut, the 18th brightest star in the night sky, and one of the Sun’s nearest stellar neighbors. The other team appears to have imaged three planets around a more obscure star known as HR 8799.The Fomalhaut planet was imaged by Paul Kalas (University of California, Berkeley) and his colleagues. Kalas and his team used the Hubble Space Telescope, which comes with an occulting disk that was employed to block Fomalhaut’s blazing pinpoint of light. Observations taken over several years revealed an ultra-faint moving object orbiting at a large distance from Fomalhaut.
photo:Fomalhaut is by far the brightest star in the dim constellation Piscis Austrinis. For observers at mid-northerly latitudes, it shines at 1st magnitude in the southern sky during autumn evenings. The star is 25 light-years from Earth. It belongs to spectral type A3, and is about 15 times more luminous than our Sun.
The purported planet orbits Fomalhaut at a whopping 119 astronomical units (1 a.u. is the average Earth-Sun distance). This puts it four times farther from Fomalhaut than Neptune is from the Sun. The planet, known as Fomalhaut b, orbits just inside a dusty ring of rubble that is Fomalhaut’s equivalent of our Kuiper Belt.Kalas and his colleagues cite two lines of evidence to argue that Fomalhaut b is indeed a planet. First, its extreme faintness in visible light, coupled with Fomalhaut’s estimated 100- to 300-million-year age, argues that it cooled off too quickly to be a brown dwarf, and thus has at most 2 or 3 times the mass of Jupiter. Kalas also points out a second piece of evidence: "A brown dwarf could not sit so close to the belt without completely disrupting it by gravity."
photo:This illustration compares the Fomalhaut system to our solar system. Both systems have dusty disks far from the star, but Fomalhaut's disk is much farther out. The disks come from colliding comets grinding themselves to dust. Fomalhaut's disk is analogous to our Kuiper Belt.
Given these two completely independent lines of evidence that Fomalhaut b has a very low mass, I’m buying the argument that Fomalhaut b is a genuine planet. But there is still no universally accepted definition of what distinguishes a high-mass planet from a low-mass brown dwarf. According to some astronomers, the line should be drawn at about 13 Jupiter masses — the mass at which a gaseous body can briefly fuse deuterium atoms in its core. According to that definition, Fomalhaut b is clearly a planet.
But other astronomers think the distinction should be based on formation. If a 3-Jupiter-mass object formed like a star from a collapsing gas cloud, it’s a very-low-mass brown dwarf. If it formed inside a disk, then it’s a planet. Since it’s unclear how Fomalhaut b formed, one could argue either way.The HR 8799 planets were imaged by a team led by Christian Marois (Herzberg Institute of Astrophysics, Canada). This group used the 10-meter Keck II telescope in Hawaii and the 8-meter Gemini telescope in Hawaii to image three pinpricks of infrared light orbiting HR 8799, a magnitude-6 star in the constellation Pegasus. Besides using an occulting mask to blot out the star’s light, the team used adaptive optics to compensate for the blurring effects of Earth’s atmosphere.
photo:In this image from the Keck Observatory, the light of the star HR 8799 has been blotted out by an occulting disk to reveal three faint companions, color coded as red dots. Previous positions are indicated, which allows us to see orbital motions. The system appears to be nearly face on, and the planets are all moving in the same direction and in approximately the same plane.
Christian Marois (Hertzberg Institute) / W.M. Keck Observatory, and others
But are these pinpricks of light actually planets? Based on their separations from the star and HR 8799’s measured 128-light-year distance, the bodies orbit at distances of about 24, 38, and 68 a.u. The innermost object would be halfway between Uranus and Neptune in our solar system, and the outermost would be slightly more than twice Neptune’s distance.Based on the infrared luminosity of the three companions, and the star’s estimated 60-million-year age, Marois and his team estimate the masses to be around 10, 10, and 7 Jupiters, respectively. These masses are getting uncomfortably close to the 13-Jupiter-mass lower limit for brown dwarfs. Moreover, the star’s age is not known to high precision, and astronomers have not thoroughly tested the cooling models they use to determine the masses of brown dwarfs and planets. In other words, the actual masses might exceed 13 Jupiters.
photo:This sky map shows the location of HR 8799. The star is just barely visible to the naked eye under very dark skies. The star is spectral type A5, and is about 5 to 10 times more luminous than our Sun. It is located 128 light-years from Earth. By coincidence, it lies in nearly the same direction as 51 Pegasi, the first solar-type star discovered to have a planet.
But as codiscoverer Bruce Macintosh (Lawrence Livermore National Laboratory) points out, "All three of these objects seem to be orbiting in the same plane, and they’re going around in the same direction. This would imply they formed in a protoplanetary disk, like planets do."I’m ready to buy that argument, at least for now. With three substellar companions moving in the same direction and in the same plane, the HR 8799 system looks like a scaled-up version of our own solar system. It looks a heck of a lot more like a planetary system than it resembles a multiple-star system.
I’d like to see both systems given further scrutiny so astronomers can better characterize the orbiting companions. I would also like to see detections around other stars, so we can start comparing different systems. But if I had to bet, I’d put my money on the claims that these are indeed planets. If Kalas, Marois, and their colleagues are right, they may go down in the history books as having taken the first images of extrasolar planets.
photo:This diagram compares the HR 8799 system (left) to our solar system (right). Like Fomalhaut, HR 8799 is surrounded by a dusty disk. The outermost planet is just inside the disk, just as Fomalhaut b lies just inside its disk. Both planets appear to be gravitationally sculpting their disks, but are not massive enough to destroy it. Click on the image for a larger view.
Regardless of the uncertainties in formation and semantics, these direct images represent a giant leap forward. "These discoveries are extraordinarily exciting for exoplanet science," says veteran exoplanet hunter Geoff Marcy (University of California, Berkeley). "We may be witnessing the birth of a new exoplanet era. For the first time, we may measure orbits, brightnesses, and spectra of other planets, just as astronomers have done for decades with stars, nebulae, and galaxies."
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