Exoplanet in Action

Yesterday the European Southern Observatory issued a press release containing an incredible image of an alien planet moving from one side of its host star to the other in just 6 years. The star, known as beta Pictoris, has been studied for several decades after the early discovery that it was surrounded by a debris disk. With new technologies that now enable high-resolution imaging from large ground-based telescopes, we can actually watch an exoplanet as it moves around in orbit.

In 1983, beta Pictoris was one of four hot stars (including also Vega, Fomalhaut, and epsilon Eridani) that were discovered to be surrounded by disks of gas and dust using the IRAS satellite. The classic image of beta Pictoris at the center of a cross-hair with the debris disk shooting out diagonally in 1980's graphics instantly became an icon -- symbolic of the quest for extra-terrestrial life. If we could see an alien solar system being formed before our eyes, then the chances seemed good that we are not alone in the Universe. It was just 18 months ago that NASA released the first direct image of an exoplanet from the Hubble Space Telescope, in orbit around one of the other IRAS stars, Fomalhaut. But in that case the orbit of the planet was 10 times the size of Saturn's orbit around the Sun -- so the planet only moved a tiny fraction of its orbit in the two years between the sets of observations. The planet around beta Pictoris is much closer to its star, at a distance between the orbits of Saturn and Uranus in our solar system.

Soon after the Hubble announcement in 2008, a team of French scientists released their first image of a faint object close to the star beta Pictoris, from observations made at the Very Large Telescope in 2003. The telescope has a computer-controlled mirror that can actively deform itself to compensate for the distortions caused by turbulence in the Earth's atmosphere. Using this technology, the scientists were able to cleanly separate the image of the faint object from the image of the bright star, just one ten-thousandth of a degree apart in the sky (roughly the size of a mosquito viewed from a mile away). The trouble was, they couldn't rule out the possibility that the faint object was a distant background star -- so they had to wait several years to see if the object would move. In late 2009 they obtained a second high-resolution image, and sure enough the faint object had moved to the other side of beta Pictoris. It was a true planet, about 9 times as big as Jupiter.

The astronomers estimate that they should be able to see the planet go all the way around beta Pictoris in less than 20 years. By then this type of observation should be routine, and it would be surprising if we weren't monitoring the movements of hundreds of such exoplanets.