A Bounty of Midsize Planets Is Reported

Credit...European Southern Observatory

There is a lot of new territory out there in the cosmos, but nothing you would want to pitch camp on  yet.

About a third of all the Sun-like stars in our galaxy harbor modestly sized planets, according to a study announced Monday by a team of European astronomers.

At a meeting in Nantes, France, Michel Mayor of the Geneva Observatory and his group presented a list of 45 new planets, ranging in mass from slightly bigger than Earth to about twice as massive as Neptune, from a continuing survey of some 200 stars.

All of the planets orbit their stars in 50 days or less, well within the corresponding orbit of Mercury, which takes 88 days to go around the Sun, and well within frying distance of any lifelike creatures.

Among the bounty is a rare triple-planet system of “super-Earths” around the star HD 40307, about 42 light-years away in the constellation Pictor. The planets are roughly four, seven and nine times the mass of Earth and have orbital periods of 4, 10 and 20 days, respectively.

Dr. Mayor called the discoveries “only the tip of the iceberg” in a news release from the European Southern Observatory in Garching, Germany.

Theories of planet formation, Dr. Mayor said in an e-mail message from Nantes, hold that smaller planets like super-Earths and Neptunes should be numerous. “But evidently it was a nice surprise to see that with our instrument we have the sensitivity to detect that population,” he said.


Sara Seager, a planetary theorist at the Massachusetts Institute of Technology who was one of the organizers of the Nantes conference, said in an e-mail message, “We’ve always been hoping that low-mass planets are common  to increase the chance for an Earth analog to exist around a nearby star.”



Dr. Mayor and his team discovered the first so-called exoplanet orbiting a regular star, known as Pegasi 51, in 1995. That planet, about half the mass of Jupiter, circles its star tightly in a four-day orbit. In the years since, some 270 exoplanets have been discovered, many of them like the original, so-called hot Jupiters in lethal scorching embraces of their stars.

Part of the reason that such unusual systems have been found first is that the detection method is biased toward finding large planets close to their stars. Both Dr. Mayor’s and Dr. Marcy’s groups use what is called the wobble method, deducing the presence of a planet by the to-and-fro gravitational tug it gives its star as it orbits. The more massive the planet and the closer it is, the bigger and more noticeable tug it will impart.

The tug perturbs the star’s velocity relative to Earth by as little as a few meters per second in the case of a super-Earth. That shows up as a periodic shift in the wavelength of light from the star.

In recent years, Dr. Mayor’s group has used a special spectrograph known as Harps, for High Accuracy Radial Velocity Planet Searcher, on a telescope at the European Southern Observatory’s site at La Silla, Chile, to detect such small wobbles in stars.

“Detection of planets with masses of about 2 Earth-masses (maybe less!) is possible,” Dr. Mayor said in an e-mail message.

To do much better, astronomers will have to go to space.

About one in 14 stars harbors a massive giant planet like Jupiter or Saturn, Dr. Mayor estimated. If in fact one in three harbors a Neptune or super-Earth, that is an appealing situation for astronomers and others who would like someday to find someplace livable or even someone living Out There.

Dr. Seager compared the quest to a giant Sudoku game. “Every time we fill in a key number,” she said in an e-mail message, “we take a big step towards finishing: finding the habitable planets and understanding how rare or common our solar system actually is.”