Seen, Not Inferred: Exoplanets Galore

While all of us who are rooting for the existence of little green men have been cheered by each discovery of a planet orbiting a star other than our sun—an “exoplanet,” of which there were 322 when I checked the catalog a minute ago—there’s always been a tinge of disappointment. Every validated discovery, starting with the first in 1995, has been indirect. In other words, astronomers didn't actually see the planet beyond our solar system, but instead inferred its existence by, for instance, noticing something funny about how a star moves and realizing, gee, that funny movement must be due to a planet tugging gravitationally on the star. But this afternoon, two separate teams of astronomers, using three different telescopes, are announcing the discovery of exoplanets by, well, looking.

One team, led by Paul Kalas of the University of California, Berkeley, used the Hubble Space Telescope to image a planet they call Fomalhaut b, orbiting the star Fomalhaut, 25 light years away in the constellation Piscis Australis (the Southern Fish). The other team, anchored by Christian Marois of the Herzberg Institute of Astrophysics in Victoria, British Columbia, imaged three planets orbiting a star called HR 8799, 128 light years from Earth, using the Keck and Gemini telescopes. Both are being published this afternoon online by the journal Science, at its Science Express website.

Typically, astronomers infer the existence of a planet around a star by studying how the star moves toward and recedes from Earth. If the motion is unusual, it implies the existence of a nearby object pulling on its gravitationally. That has let astronomers calculate the mass and orbit of the inferred planet. Only six exoplanets have been detected by actual imagery, and there are doubts about several—specifically, whether they are true planets or brown dwarfs, which are basically failed stars and not planets at all. Also, all of the six orbit at such large distances from their star that astronomers doubt they formed the same way the planets of our solar system did, which makes them less interesting for anyone hoping to find Earth-like, life-supporting worlds beyond the Sun.

This discovery breaks that mold. The Gemini telescope first spied two planets around HR 8799 on October 17, 2007. Eight days later, and again this past summer, Marois and colleagues used the Keck II telescope in Hawaii  to see a third planet orbiting even closer to the star. “Until now, when astronomers discover new planets around a star, all we see are wiggly lines on a graph of the star’s velocity or brightness,” said Bruce Macintosh of Lawrence Livermore National Laboratories. “Now we have an actual picture showing the planets themselves, and that makes things very interesting. . . . After all these years, it’s amazing to have a picture showing not one but three planets. The discovery of the HR 8799 system is a crucial step on the road to the ultimate detection of another Earth.”

The planets are about 10, 10 and 7 times the mass of Jupiter, orbiting HR 8799 (which is 50 percent larger and 5 times brighter than our Sun) at about 24, 37 and 67 times the distance between Earth and Sun, respectively. “I think there’s a very high probability that there are more planets in the system that we can’t detect yet,” Macintosh said. “One of the things that distinguishes this system from most of the extrasolar planets . . . is that HR8799 has its giant planets in the outer parts, like our solar system does, and so has room for smaller terrestrial planets, far beyond our current ability to see, in the inner parts.”

The discovery by the Hubble is of a planet estimated to be no more than three times Jupiter’s mass, and about 10 times the distance between Saturn and the Sun. Its discoverers plan to observe the planet in infrared light, which might reveal the existence of water vapor clouds in the atmosphere. If they find water, it would be more evidence that the requirements for life—a nice star, a decent-sized planet the right distance away, and water—are not that rare in the Milky Way galaxy.