When first discovered over a decade ago they appeared as lonely giants, circling their home stars in splendid isolation amid the emptiness of interstellar space. Then, as more and more exoplanets were detected, some of them clustered together in groups of twos and threes orbiting the same star. And now, at last, a fully formed planetary system, with five different planets of varying sizes and orbits has been found, orbiting a star more than 40 light years away. Significantly, it is the very same star, 55 Cancri, that was one of the chief targets of the SETI@home reobservations at Arecibo in March 2003.
The discovery was announced on November 6 by the California and Carnegie Planet Search Team, a group of veteran planet hunters responsible for detecting more than half of the exoplanets known to date. The group has been observing 55 Cancri, for 19 years now, gathering radial velocity data in search of the slight wobbles that indicate the presence of orbiting planets. Their persistence is clearly paying off. The exceptionally long observation record has enabled the researchers to detect the telltale signs of orbiting planets one by one in the data, and there may still be more to come. “We probably still don’t have all the planets” said team member Debra Fischer of San Francisco State University, who is lead author on the forthcoming article on the recently discovered fifth planet. “We are pulling out one thread at a time, disentangling all these orbits” she added; “I think it’s amazing what we’ve been able to do with the system."
55 Cancri, as its name suggests, is the 55th brightest star in the constellation Cancer. It is similar in mass to our Sun, though older and somewhat dimmer, and it is located only 41 lightyears away – practically in our galactic back yard. Remarkably, as the outlines of the star's planetary system gradually emerged, it became clear that its basic architecture resonates strongly with that of our own solar system. 55 Cancri's system includes one giant planet, four times the mass of Jupiter, orbiting at a Jupiter-like 6 astronomical units (AU) from the star, and four smaller planets orbiting inside 0.8 AU, with an enormous gap in between. Compare that with our own solar system, where giant Jupiter orbits at an average distance of 5.2 AU from the Sun, and the four smaller inner planets are all congregated within 1.5 AU from the Sun.
Researchers find such similarities suggestive. The giant gap in the 55 Cancri system between the outermost giant planet and the one next in, notes team leader Geoffrey Marcy of U.C. Berkeley, is where one would expect to find Earth like rocky planets. In fact, Fischer notes, any planet within this gap must be relatively small, or it would destabilize the orbits of the known planets. And while the detection of such distant Earths is currently beyond the capacity of planet hunting technology, Marcy believes that this will soon change. Within five years, he predicts, continued observations may reveal the presence of rocky planets around 55 Cancri.
Despite the obvious similarities, notes Marcy, “we haven’t found a twin of our solar system, because the four planets close to the star are all the size of Neptune or bigger.” The first of these was discovered back in 1996, and was only the fourth exoplanet to be detected anywhere. Like most of that first batch of exoplanets it was a “hot Jupiter,” a gas giant orbiting in the very shadow of its star, completing each orbit in only 14.6 days. Next, in 2002, came the “super Jupiter” at 6 AU, which circles its star once every 14 years, and later the same year came the third planet, a “mini Saturn” with a 44 day-long “year.” The fourth planet, discovered in 2004, is the smallest of the group at 14 Earth masses, or about the size of Saturn. But with an orbital period of just 2.8 days it orbits so close to its star that its surface is practically scorched, negating any possible analogies with our home planet.
The newly discovered fifth planet, whose detection spurred the announcement, is about half the mass of Saturn, orbits its star at an average distance of 0.785 AU, and completes each revolution in 260.8 days. Intriguingly, this puts it right within 55 Cancri’s “habitable zone,” the region around a star where liquid water is stable, and life as we know it at least conceivable. This may not be significant for the new planet itself, which is after all a gas giant incapable of holding liquid substances. But, noted Fischer, “the gas-giant planets in our solar system all have large moons; if there is a moon orbiting this new, massive planet, it might have pools of liquid water on a rocky surface.”
This is not the first time that 55 Cancri has been mentioned as a target for the search for life in the universe. Back in March 2003 Dan Werthimer’s SETI@home crew set out to Arecibo to revisit the most promising locations in the sky for extraterrestrial communications. At Marcy’s recommendation they included several stars known to be home to planets or planetary system, and topping the list was none other than 55 Cancri. Even at the time, with only the three known planets orbiting the star, the system around 55 Cancri was considered a close kin to our solar system, and therefore a good candidate for the search for intelligent life.
Sadly, despite listening closely to signals emanating from the star, Werthimer and his team could find no hint of the signature of an intelligent transmission. But with a fast growing planetary system around 55 Cancri, and with ever more sensitive equipment at their disposal, the SETI@home crew will no doubt be back for another round of observation in the not-too-distant future.
About 250 exoplanets have been discovered to date, and more are being announced at a steady rate. What began as a bizarre collection of hot Jupiters orbiting their stars at dizzying speeds soon became a more representative sample, including smaller, cooler, and more distant planets. Now whole planetary systems are emerging from the shadows, both similar and intriguingly different from the one we call home. But even, notes Marcy, is only one small step in our scientific quest. “Earth-like planets are the next destination."