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Direct Imaging

Direct imaging of exoplanets is extremely difficult, and in most cases impossible. Being small and dim planets are easily lost in the brilliant glare of the giant stars they orbit. Nevertheless, even with existing telescope technology there are special circumstances in which a planet can be directly observed.

In July 2004 a group of astronomers using the European Southern Observatory's Very Large Telescope Array imaged a planetary mass object, several times the mass of Jupiter, in close proximity to a brown dwarf designated 2M1207 at a distance of 200 lightyears from Earth. Further observations confirmed that the object was indeed in orbit around 2M1207, and many consider the images to be the first directly imaged exoplanet. Others are more skeptical: the reason the "planet" could be imaged is that it is in orbit around a brown dwarf, which is far dimmer than a true star. Since planets are supposed to be in orbit around stars, and since the object may have formed as a miniature star some do not consider it a proper planet.

First Image of an Exoplanet?

NaCo, VLT, ESO

First Image of an Exoplanet?
In July 2004 a group of astronomers led by Gael Chauvin took this image of a planetary-mass object in orbit around brown dwarf 2M1207.

In November of 2008 a group of astronomers using the Hubble space telescope announced that it had imaged a planet orbiting the star Fomalhaut. The discovery was made possible by the fact that Fomalhaut is surrounded by thick disk of gas and dust. The sharp inner edge of the disk suggested to astronomers that a planet had cleared out debris from its path, and pointed out where its orbit would be. Following up on these clues the astronomers were able to locate the planet in the Hubble images of the disk. Even so the planet, estimated at no more than twice the mass of Jupiter, might well have remained invisible were it not for the fact that it is extraordinarily bright. This led scientists to believe that it is surrounded by a ring system many times thicker and more luminous than that of Saturn.

On the same day that the imaging of the planet around Fomalhout was made public another group of astronomers announced the imaging of 3 planets orbiting the star HR 8799. Unlike Fomalhaut b, which was imaged in visible light, these planets were detected in the infrared range of the spectrum. This is because HR 8799 is a young star and the planets around it still retain some of the heat of their formation, which registers in the infrared range. Furthermore, whereas in the visible range the reflected light from the planets would be swallowed up by the brilliance of the star, the independent heat of the planets stands out far more clearly in the infrared range.

All in all, despite the difficulties, scientists have managed to produce several images of exoplanets. With today's technology, however, such imaging is possible only in rare and unusual situations.

A Distant World

NASA, ESA, and Z. Levay (STScI)

A Distant World
Hubble Space Telescope image of planet Fomalhaut b orbiting the star Fomalhaut. 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.

Advantages

For humans there is something magical about the ability to actually "see" an object. "Seeing is believing" goes the saying, and that applies to extrasolar planets as much as for anything else. There is no substitute to actually seeing a faraway planet, and in this respect direct imaging is the holy grail of planet hunting.

On the rare occasions when direct imaging is feasible, it can provide scientists with valuable information about the planet. In the case of Fomalhaut b, for example, the planet's interaction with the protoplanetary disk and the fact that it is invisible in the infrared provided strong limits to its mass, and its exceptional brilliance led scietists to theorize that it is surrounded by a massive ring system. In the case of HR 8799, the infrared radiation from the objects combined with models of planetary formation provides a rough estimate of the planets' mass.

For obvious reasons, direct imaging works best for planets that orbit at a great distance from their stars, and for planetary systems that are positioned "face on" when observed from Earth. This makes it complimentary to the radial velocity method, which is most effective for planetary systems positioned "edge-on" to Earth and planets orbiting close to their parent star.

Three planets around HR 8799

Gemini Observatory, artwork by Lynette Cook

Three planets around HR 8799
Artist Lynette Cook's conception of the three planets imaged orbiting star HR 8799.

Disadvantages

With current observation technology direct imaging is possible on on very rare occasions. It is most likely to succeed when conditions are just right, namely when a bright planet orbits at a great distance from a nearby star. Because of these strict limitations direct imaging is not a good candidate for large-scale surveys searching for new exoplanets. For the forseeable future directly imaged planets will remain very much the exception among known exoplanets rather than the rule. The method's importance today is as much psychological as it is scientific.

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