Groups take first photos of extrasolar planets

By: Husain Danish

Posted: 11/20/08

Astronomers have taken the first direct images of planets outside of our solar system. The visible images were taken of planets orbiting the star Fomalhaut, located 25 light years away from Earth in the constellation Piscis Austrinus.

In a separate study, an exoplanetary system comprised of three planets circling a star 130 light-years away in the constellation Pegasus, was directly imaged. These findings were published online in the journal Science.

None of the four planets are remotely habitable, nor do they resemble Earth. In fact, the four gas giants are much more similar to Jupiter. However, scientists have not ruled out the possibility that either planetary system contains Earth-like worlds.

The search for extrasolar planets has, until now, employed two techniques. Many astronomers have focused on detecting the "wobble" that planets induce on their parent star. Stars and planets, like all objects with mass, exert a gravitational force on other objects.

While the gravitational force exerted by stars keeps planets in their orbit, the gravitational force exerted by planets causes their parent stars to wobble in place. This movement can be observed by examining the light coming from the stars.

Scientists could also wait for the planet to transit or pass in front of its star. A planetary transit would result in a temporary, detectable decrease in the star's brightness. Most planets discovered using these techniques have been very large, many larger than Jupiter.

While these techniques have led scientists to discover over 300 extrasolar planets, scientists have struggled to directly image these planets. These recent images are very significant. For the first time, a planet, rather than the effects of a planet, have been observed. Images would be valuable in determining the composition and atmosphere of extrasolar planets. The same technology could be applied in the search of new Earth-like planets.

However, there are two major problems that need to be overcome: Earth-like planets will most likely be orders of magnitude fainter than gas giants like Jupiter. Second, terrestrial planets will be located much closer to their parent star, making them more difficult to image.

The problem for years was that there was too much light: Light emitted from a star would simply cover up any light emitted from a planet orbiting it. The problem is similar to finding a burning match in a forest fire. However, recent advances in imaging and optics have allowed scientists to effectively remove the bright light emitted by stars, leaving behind only the light reflected by the planets.

An international team, led by Paul Kalas of the University of California, Berkeley, used the Hubble Space Telescope to take a visible-light image of the region around the star Fomalhaut. The star has a massive ring of dust surrounding it that appears to have a cleanly groomed inner edge.

It was an ideal candidate to look at, because it fits the accretion model, a theory that proposes that planets form when large masses orbiting around stars begin gathering up nearby debris. As the debris accumulates and condenses, primordial planets increase in size.

The team estimates that the planet, named Fomalhaut b, is 18 billion kilometers away from its star and is about three times the mass of Jupiter. Fomalhaut b completes an orbit in about 870 years and may have a ring system similar to one Jupiter had in its earlier history.

Bruce Macintosh, an astrophysicist from Lawrence Livermore National Laboratory in California, led a separate team that imaged a planetary system orbiting a star named HR 8799, located in the Pegasus constellation. Using the Gemini North telescope and W.M. Keck Observatory on the island of Mauna Kea in Hawaii, the astronomers obtained infrared images of the three member planetary system.

The solar system is similar to our own. The three planets, which are estimated to be between seven and 13 times the mass of Jupiter, are as far away from their parent star as the outer planets in our solar system. The smaller planets are also closer to their star than the larger planets, an observation predicted by accretion theory.

Astronomers estimate the planets to be only about 200 million years old, young compared to the Earth, which is over four billion years old. These planets provide astronomers with an opportunity to study the early stages of planet formation and solar system evolution.

However, there is some dispute about whether or not these images are actually of extrasolar planets. Earlier claims have been made, and some have yet to be confirmed. Many have turn out to be brown dwarfs, sub-stellar objects whose mass is below the necessary limit to maintain a nuclear fusion reaction.

"My understanding is that at least one of them (the one seen by Hubble) has a dust ring near the planet and that the astronomers determined that the planet must have a mass less than three times that of Jupiter or it would have disrupted the dust disk," Hopkins physics and astronomy professor Adam Riess said.

"Three times Jupiter is too small to be a brown dwarf and really is a planet. The definition usually involves being massive enough to have any kind of fusion. ?Deuterium fuses at approximately 13 times the mass of Jupiter, so this guy qualifies as a planet."