In the old days — which is to say, the 1990s — discovering a new planet orbiting a distant star was enough to keep an astronomer in the news for days, and maybe even lead to a cover story in TIME. Nowadays, with the extrasolar planet count well into the 400s, even finding an entire alien solar system, while not exactly routine, is not unheard of.
This week, in fact, it's been heard of twice. On Tuesday, a team of stargazers using the European Southern Observatory in the high Chilean desert announced they'd detected a system of at least five, and maybe as many as seven, planets circling a star known as HD 10180, about 127 light-years from Earth, in the direction of the constellation Hydrus. And just two days later, a paper appeared in Science trumpeting the discovery of a multiplanetary system circling a star called Kepler-9, 2,000 light-years away in the constellation Lyra. The latter solar system has only two or three worlds — but the space telescope that found it is so powerful that this discovery is just a hint of the other worlds and other solar systems it may discover in the next few months. (See pictures of five nations' space programs.)
Both detections are scientific tours de force in different ways. In the first, scientists found the planets indirectly, by noting how HD 10180 is being tugged back and forth by a swarm of circling planets. That's how the first extrasolar planets were found in the mid-1990s, but the effect is so subtle that doing a clear analysis of the mass and orbit of even a single planet is tough. Untangling multiple, independent, overlapping sets of wobbles is excruciatingly hard. (See the top 50 space moments since Sputnik.)
In this case, the untangling showed five worlds between 13 and 25 times as massive as Earth, which puts them in the general range of our own Neptune. Unlike our relatively uncluttered system, though, the five Neptunes are crammed into what we'd call the inner solar system: the most distant of the planets orbits at about the distance of Mars, with a year that lasts about 600 days. The closest is snuggled far closer to its star than Mercury is to our sun, with a year of only six days — and there are three Neptunes in between.
That's not all: the new system may also contain a Saturn-like world orbiting much farther out — and, most tantalizingly, a planet just 1.4 times larger than Earth, orbiting even closer to its star than the innermost Neptune. If it's confirmed, this will be the smallest extrasolar planet, or exoplanet, ever found and thus, the closest yet to the ultimate goal of discovering something exactly Earthlike — in size, anyway. Temperature would be a different matter since a planet that close to a star would sizzle in the thousands of degrees.
As for the Kepler-9 system, it was found by NASA's Earth-orbiting Kepler spacecraft, which looks not for wobbles but for the silhouette of distant planets as they transit or pass in front of their stars. Kepler scientists have announced only five new worlds since the probe was launched early in 2009. But this past June, they revealed that they had some 700 more candidates in the can, which will next be subjected to an exhaustive confirmation process.
This system, with two Saturn-size worlds circling their sun inside what would be Mercury's orbit, is the first time a multiple-planet system has been found this way. And as with the European discovery, this system may be home to a still unverified superhot planet, only a little bigger than Earth. (See an illustrated history of planet Earth.)
But these, say the Kepler scientists, aren't the most exciting things about the new find. One big advantage of transiting planets is that you can tell how physically large they are, not just how massive, because the amount of starlight a planet blocks is a direct measure of its size. If you know the size and if you can also figure out the mass, you know the planet's density — a powerful clue to what it's made of. A transiting planet announced last winter by Kepler, for example, has the approximate density of Styrofoam, suggesting that it's made mostly of gas. Another, found by the ground-based MEarth Project (that's not a typo; it looks for planets around stars known as M dwarfs), has the density of water — and might in essence be a gigantic water droplet.
In the latest case, the Kepler scientists were also able to get the planets' masses — but this time, they did it with a brand-new technique. As the Saturns orbit, they tug not only on their star but also on each other. As result, says the Science paper's lead author Matthew Holman, of the Harvard-Smithsonian Center for Astrophysics, their orbits are slowly changing in velocity, and that lets the scientists calculate each planet's mass.
They can't do the same yet for the tiny hot planet in the system, but further observations might nail down that much trickier measurement for this and for the other solar systems Kepler is inevitably going to find. "We've only looked at the first seven months of data," says Bill Borucki, the Kepler mission's principal investigator, and a co-author on the Science paper. "Within the next few years we should be able to give you a lot more information. Within the next few years," he adds, "we will have answers to the questions of how frequently Earth-mass planets occur, and how often they orbit in the habitable zones of their stars." It's in the habitable zone — on Earthlike worlds — that life as we know it is likeliest to be found.
