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Planetary News: Extrasolar Planets (2007)Most Earth-Like Planet Discovered Orbiting Nearby StarBy Amir AlexanderApril 25, 2007
A giant red orb in the sky shines down on planet Gliese 581c during the day, and at night the bright lights of two neighboring planets move gently through the starry skies. On the surface below the balmy landscape might be crisscrossed with bubbly streams and flowing rivers, or vast oceans might lap against sandy shores. We don’t know, and in fact we don't even know whether the planet has water at all. But we do know that among all planets discovered to date orbiting faraway stars, Gliese 581c resembles our Earth far more than any other. One reason is that Gliese 581c is the smallest exoplanet (or "extrasolar planet") ever discovered. Whereas the majority of exoplanets discovered so far are gas giants, dozens or hundreds of time more massive than the Earth and resembling Jupiter in size and composition, Gliese 581c is likely a rocky planet like our own. Its radius is just one and a half times that of the Earth, and its mass only five times greater. In the long quest to find Earth-like planets, the detection of a world so close in size to our own is in itself a notable event. But even more significant than the planet’s Earth-like dimensions is its location: Gliese 581c orbits its star within that narrow band of space known as the “habitable zone,” defined as the region around a star where liquid water is stable. This makes it the only known world outside our solar system where water, the elixir of life, could be flowing. “Liquid water is critical to life as we know it” explained Xavier Delfosse of the University of Grenoble in France who is a member of the team that discovered the Planet. “On the treasure map of the universe,” he added, “one would be tempted to mark this planet with an X.” The planet where water may flow was discovered by an international group of scientists from Switzerland, France, and Portugal, led by Stephane Udry and Michel Mayor of the Geneva Observatory. Over the past few years the group has been conducting a systematic search for low mass planets using the 3.6 meter telescope at the European Southern Observatory’s facility in La Silla, Chile. With the help of the telescope’s ultra-sensitive HARPS spectrograph (High Accuracy Radial Velocity for Planetary Searches), they are conducting a radial velocity (or "spectroscopic") search for exoplanets, looking for slight periodic shifts in a star’s spectrum. These are the telltale signs that the star is wobbling – rocking to and fro at regular intervals to the tug of an orbiting planet. Two years ago Udry and his colleagues detected such a wobble in Gliese 581, a small and dim star in our galactic neighborhood – only 20.5 light years away. With a mass only one third that of our Sun, and 50 times fainter, stars like Gliese 581 are known as a “red dwarfs,” and are the most common type of star in the galaxy. When they analyzed the observation results the researchers concluded that the star has a companion -- a Neptune-size planet of around 15 Earth masses orbiting very close to the star and completing its course every 5.4 days.
While this was news in itself, Udry and Mayor’s team also noticed that the star’s observed wobble did not quite fit the expected pattern. Additional forces seemed to be tugging at the star in addition to the known “Neptune,” and that meant, most likely, additional planets. And so, last year they returned to La Silla to collect more data and decipher the mystery of Gliese 581’s wobble. They were not disappointed: the additional observations clearly indicated that a small planet of only five Earth masses is orbiting the star every 13 days. The additional observations also strongly suggested that a third planet of around eight Earth masses was moving further away, completing each orbit every 84 days. The detection of a planetary system with three low-mass planets so close to the Earth is in itself big news. Only one other comparable system had been found so far, around HD 69830, and it too was discovered with the HARPS spectrometer. But it was the middle planet of the trio that truly caught the attention of Udry and his colleagues. Not only was it the lowest mass planet ever found, but its distance from its star indicated that conditions on its surface would be unusually temperate. Any planet orbiting so close to our own Sun would be seared by the heat, but that is not the case for the much cooler red dwarf Gliese 581: “we have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid” explained Udry. The fact that the first known exoplanet capable of sustaining liquid water was found orbiting a red dwarf star is no coincidence, said team member Xavier Bonfils of Lisbon University in Portugal. “Red dwarfs are ideal targets for the search for such planets because they emit less light, and the habitable zone is thus much closer to them than it is around the Sun.” This means that a habitable planet orbiting a red dwarf will complete each revolution much quicker than a habitable planet orbiting a Sun-like star. Whereas our Earth, moving in the Sun’s habitable zone, takes a full year to complete each orbit, Gliese 581c, circling a red dwarf, completes its course in a matter of days. This makes its detection much easier for a radial velocity search, which looks for the regular rocking to and fro of a star as its planetary companion completes each and every orbit.
Furthermore, the fact that a red dwarf is of much lower mass than our Sun also makes the detection of small rocky planets easier, explained Udry. Gliese 581c, for example, causes the star to move at a rate of around 2 to 3 meters per second -- the speed of a briskly walking human. This wobble is detectable by the HARPS spectrograph, whose sensitivity allows it to register motions as slow as 1 meter per second. The same planet orbiting our Sun would induce it to move at a rate of no more than a few dozen centimeters per second, which would make the wobble wholly undetectable by today’s technology. All in all, both the low luminosity and the low mass of red dwarfs favor the detection of Earth-like planets around such stars. With this in mind, Udry and his team are confident that the discovery of many more such planets is not far off. “We are observing thousands of stars” said Udry, “and our theoretical models are all telling us that there should be many planets among them. “The fact that we are finding them relatively easily only confirms this," he added. Mayor, who in 1995 was responsible for the discovery of the very first exoplanet is also optimistic: “we are confident,” he said, “that given the results obtained so far, Earth-mass planets around res dwarfs are within reach.” Meanwhile, 20.5 light years away, a balmy and temperate planet orbits its red sun, holding tight to its secret. Is it rich in water? Do rivers and oceans mark its surface? And, most intriguingly, is it home to any form of life? As yet, we do not know. But across the emptiness of space the mystery of Gliese 581c now beckons. Dreamers, as well as scientists, hear its call. |
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