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Planetary News: Extrasolar Planets (2008)

Scientists Lay Eyes on Distant Planets

By Amir Alexander
November 14, 2008
A Distant World
A Distant World
Hubble Space Telescope image of planet Fomalhaut b orbiting the star Fomalhaut. It is the first visible light image of an exoplanet. Scientists imaged the planet by using a coronagraph that blocks out the star and it immediate vicinity. This accounts for the dark region at the center of the image. Credit: NASA, ESA, and Z. Levay (STScI)

The astronomical community is abuzz this week following twin announcements that scientists have finally managed to directly image planets orbiting distant stars. The announcements came more than thirteen years after the detection of the first exoplanet, and follows the discovery of around 300 additional distant planets in the years since then.

While all the planets discovered to date differ from each other in many crucial respects, they all have this in common: they are known to scientists indirectly, their presence established through mathematical curves rather than direct visual evidence. An exoplanet could be known by the sinusoid graph that characterizes the subtle motion of star rocking to the tug of an orbiting planet, or by the telltale drop in a star's luminosity as a planet transits between it and the Earth. But never has a planet outside our solar system been captured through the lens of a telescope and presented directly to human eye. Never, that is, until now.

In two papers published on November 13, 2008 in the journal Science Express scientists describe the imaging of four planets around two young stars designated Fomalhaut and HR 8799. The single planet found orbiting Fomalhaut was imaged in visible light with the Hubble Space Telescope's Advanced Camera for Surveys. A system of three planets orbiting HR 8799 was imaged in the infrared range by the Gemini and the Keck observatories, both in Hawaii. Even normally sedate planetary scientists were moved by the accomplishment: "It's a profound and overwhelming experience to lay eyes on a planet never before seen" said U.C. Berkeley astronomer Paul Kalas, lead author of the paper on the Fomalhaut imaging.

Fomalhaut is a young A type star 27 light years from Earth, only around 200 million years old and many times hotter and more luminous than our Sun. Being so bright it would not normally qualify as a good candidate for detecting orbiting planets, since its light would drown out a nearby planet making it practically invisible from Earth. Fomalhaut, however, is a special case, because since the early 1980's astronomers have known that it is surrounded by a thick whirling disk of rock and dust, similar to the one that enveloped our Sun in the formative years of our solar system.

In 2004 Kalas and his colleagues used Hubble to image Fomalhaut, confirming the presence and nature of the dust belt surrounding the star. But they also noted some unusual characteristics: The belt, they found, was not centered on the star, but on a point some distance from it. Furthermore, the inner rim of the belt did not peter out gradually, but terminated in a sharp edge. Both characteristics, they noted, could be most easily explained by the presence of a planet inside the inner rim, clearing a path for itself and shepherding the belt's debris along its orbit.

With this clue in mind, Kalas and his colleagues carefully examined the Fomalhaut images in search of possible planets. In the 2004 images they noted several clumps that could potentially be identified as planets, but without a clear indication of their motion there was no way to determine what the objects actually were. But when an additional set of images taken 21 months later were also examined, it became apparent that one of the candidate objects had moved ever so slightly relative to the star. It was, by all appearances, a planet, orbiting Fomalhaut at a distance of 17 billion kilometers, equivalent to ten times the distance of Saturn from the Sun. It such a distance it takes the planet more than 800 years to complete each orbit.

Although the orbiting object certainly appeared to be a planet, Kalas and his collaborators had more work to do before they could announce their discovery. Before scientists can determine with certainty whether a dim object orbiting a star is in fact a planet or simply small star, they must first estimate the candidate planet's mass. For the vast majority of extrasolar planets known to date this estimate is made from the motion of the star as rocks back and forth, towards Earth and away from it. But for the new object, now designated "Fomalhaut b", such a measurement would be impossible, because its orbital plane is "face on" to Earth rather than "edge-on," eliminating the characteristic rocking motion towards and away from the Earth. Furthermore, even if that were not the case, it would take observers more than 800 years to record a single cycle of the star's motion, making this method highly impractical.

The Hubble Space Telescope
The Hubble Space Telescope
View of Hubble captured by astronauts from the space shuttle Columbia during mission STS-109, March 2002. Hubble was used in imaging the planet orbiting the star Fomalhaut. Credit: NASA

Deprived of these traditional methods, the scientists went back to look at Fomalhaut's imposing dust belt. If the orbiting object were massive enough, they reasoned, it would not only delineate the inner edge of the disk but would push it further away from the star or disperse it entirely. Since this is not the case, they calculated that the object could not be more than three times the mass of Jupiter, or else it would disrupt the dust belt. Most planetary scientists cap planetary masses at 13 Jupiters, and so Fomalhaut b, with a maximum mass of three Jupiters, easily qualifies as a planet.

The imaging of a planet in visible light around Fomalhaut came as a surprise to Kalas and his colleagues, who thought that an image in the infrared range was more likely to succeed. This is because planets in a young system like Fomalhaut are still hot from their formative period, and so radiate strongly in the infrared. In visible light, by contrast, the planet only reflects radiation from its star, and is easily drowned out by its brilliance.

But what the team found in the case of Fomalhaut b was surprising. For one thing the planet could not be detected at all in the infrared range, which means that it is a relatively small object that radiates weakly. This was further evidence that Kalas and his colleagues' were correct in concluding that Fomalhaut b was indeed an object of planetary mass. According to team member Eugene Chiang of U.C. Berkeley this consideration further limits the mass Fomalhaut b to two Jupiters or less.

In the visible light, however, the planet shone much brighter than its mass would lead scientists to expect. Why that is is unknown, but one likely explanation is that the planet is surrounded by brilliantly reflective rings that make it shine more brightly far more brightly when seen from Earth. "If we're seeing light in reflection," Kalas concluded, "then it must be because Fomalhaut b is surrounded by a planetary ring system so vast it would make Saturn's rings look pocket-sized by comparison."

As Kalas points out, the character and location of the planet suggests a planetary system around Fomalhaut similar, though much larger, than our own solar system. This is because the giant planet orbits at a great distance from the star, much like Jupiter and Saturn but unlike the majority of known giant exoplanets, which orbit at dazzling speeds very close to their star. Furthermore, this arrangement of the Fomalhaut system allows for the presence of small rocky planets in stable orbits closer to the star, just as Venus, Earth, and Mars move inside the orbit of giant Jupiter. "There is plenty of empty space between Fomalhaut b and the star for other planets to happily reside in stable orbits" Kalas concluded.

A very similar architecture prevails among the three planets that were imaged orbiting the star HR 8799. Much like Fomalhaut this is a young A type star, one and a half times more massive than the Sun and five times more luminous, and it too is surrounded by a massive dust belt. In fact, HR 8799's dust disk is so large and thick that according to one of the Science Express article's authors, Ben Zuckerman of UCLA, that it "stands out as one of the most massive in orbit around any star within 300 light years of Earth." Unlike Fomalhaut b, however, and more in line with expectations, the planets around HR 8799 were imaged not in visible light but in the infrared range, taking advantage of the planets' heat radiation left over from their formation.

Three planets around HR 8799
Three planets around HR 8799
Artist Lynette Cook's conception of the three planets imaged orbiting star HR 8799 Credit: Gemini Observatory, artwork by Lynette Cook

The first two planets around HR 8799 were found by astronomers at the Gemini Observatory in images taken on October 17, 2007. Then, on October 25, 2007, and in the summer of 2008 additional observation with the Keck II telescope confirmed the discovery and added an additional planet, bringing the total number of imaged planets to three. Based on models that take into account the planets' age and heat radiation, all three are massive: Two of the planets are estimated at around 10 Jupiters, and the third at around 7 Jupiters. All of them, furthermore, orbit at great distances from their stars, approximately 25, 40, and 70 astronomical units (AU), with each AU designating the average distance of Earth from the Sun.

As with the case of Fomalhaut, giant planets orbiting at great remove from their star bode well for the presence of smaller rocky planets closer inside. "I think there's a very high probability that there are more planets in the system that we can't detect yet" said Bruce Macintosh of the Lawrence Livermore National Laboratory and one of the authors of the Science Express paper, adding that these may well turn out to be small terrestrial planets. "After all these years it's amazing to have a picture of not one but three planets" Macintosh said. "The discovery of the HR 8799 is a crucial step on the road to the ultimate detection of another Earth."

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