At an international conference on extrasolar planets being held in Portugal, a group of European astronomers unveiled on Monday a list of 30 new exoplanets and two brown dwarfs orbiting more-or-less Sun-like stars. This brings the known exoplanet catalog to a total of 403 worlds.
The new additions were all found by the radial-velocity wobbles that they induce in their host stars, as detected by the HARPS planet-hunting spectrograph on the European Southern Observatory's 3.6-meter telescope at La Silla, Chile.
The European astronomers say that HARPS can measure a star's radial-velocity patterns with an accuracy as fine as 1 meter per second: slow walking speed. This would put HARPS at the head of the accuracy list in the hotly competitive world of exoplanet hunting. Such precision is essential for detecting relatively low-mass planets — not just "super-Jupiters," "Jupiters," and "Saturns" of other stars, but the "Neptunes" and "super-Earths" that are at the limits of current technology.
According to a press release from the European Southern Observatory about the new finds, "HARPS has facilitated the discovery of 24 of the 28 planets known with masses below 20 Earth masses. As with the previously detected super-Earths, most of the new low-mass candidates reside in multi-planet systems, with up to five planets per system."
In particular, says Stephane Udry (Geneva Observatory), the HARPS project suggests that at least 40% of solar-type stars have these smaller planets. "These low-mass planets are everywhere basically."
Limits of Technology
To extract the very slight, periodic radial-velocity changes in a star that signify an orbiting planet, astronomers have to subtract out the much larger continual changes caused by the telescope's own motion on the rotating Earth, Earth's curving motion around the Sun (at about 30,000 meters per second), and even the gravitational influences of the Moon and our solar system's other planets on Earth. All these effects are precisely known. But there may be trickier confounding factors on the distant star itself, such as surface turbulence or starspots that mimic a change in the star's radial-velocity signature as the star's rotation carries the spots around.
Nevertheless, astronomers don't think they've yet hit the fundamental limits of the radial-velocity method for finding planets. For particularly good "quiet" stars, it may be possible to reach accuracies measured in centimeters per second, and thus planets as low-mass as Earth. Such hunts could become possible as early as next year.
Selections from the Harvest
What are the new planets and their stars like? The stars, according to a preliminary list, range from spectral type F6 to M: from somewhat larger and hotter, to much cooler and dimmer than the Sun. The orbiting objects have minimum masses (not necessarily the true mass, but probably not too far off) ranging from 53 down to 0.017 Jupiters. That's from 17,000 down to 5 Earth masses. Their announced orbital periods range from 4 days to 13 years (though HARPS has only been working for five years).
Of special note, three of the planets orbit stars with a lower proportion of heavy elements than are in the Sun — with a "metallicity" of only 1/3 to 1/2 the Sun's. It's well known that the greater a star's metallicity, the more likely it is to have planets detectable. A subgroup of the European astronomers targeted low-metallicity stars in particular, and their finds confirm that planets do sometimes form in a moderately heavy-element-poor environment. Clearly more than this one factor is at work in determining whether a star will have planets, even big ones.
Several systems also show radial-velocity hints of additional objects in longer-period orbits that will require years of further tracking to confirm.
Here's the ESO press release.
Here's the Extrasolar Planets Catalog reordered so the new discoveries form the top of the list.
P.S.: Don't expect the total to stay near 400 for too long. The Kepler mission should announce a whole flotilla of transiting exoplanets in the coming months.