NASA's Kepler spacecraft, hunting for distant worlds by measuring the slight dimming of starlight as planets pass in front of their parent suns, has found its first multiplanet solar system, researchers announced Thursday.
The Kepler-9 system includes two Saturn-class worlds orbiting in gravitational lockstep close to their star and a possible third planet just a bit larger than Earth that whirls through a hellish "year" in just 1.8 days.
The announcement came just a few days after a European team, using a different technique with a ground-based telescope, revealed the discovery of a solar system with up to seven planets, including another candidate planet slightly larger than Earth.
The combined results demonstrate a steadily improving ability to detect Earth-size worlds across vast gulfs of space, raising hopes than within a few years, scientists will know whether Earth-like planets are common or rare.
"The Kepler mission has discovered two new planets orbiting the same star and actually there's a third possible, or candidate, planet that may be transiting that star as well," said William Borucki, the Kepler principal investigator. "It is the first discovery of (multiple) planets transiting the same star.
"The observation will give us new information about a system of orbiting planets that's quite different from our own solar system. It gives us clues about the composition of these planets and actually helps us to search for other planets in the planetary system even though the planets don't transit."
Trailing the Earth in its orbit around the sun, Kepler's 95-megapixel camera is aimed at a patch of sky in the constellation Cygnus that's the size of an out-stretched hand, a target zone that contains more than 4.5 million detectable stars.
Of that total, the science team has selected some 300,000 stars that are the right age, have the right composition and brightness to host Earth-like planets. Over the life of the mission, more than 156,000 of those will be actively monitored by Kepler.
The spacecraft's camera works like a photometer, continually monitoring the brightness of target stars in a wide field of view. If a planet passes in front of a target star, the light observed by Kepler will dim ever so slightly.
By studying those subtle changes in brightness--comparable to watching a flea creep across a car's headlight at night--and by timing repeated cycles, computers can ferret out potential Earth-like worlds in habitable-zone orbits even though the planets themselves cannot be seen.
The probability of finding sun-like stars with Earth-like planets in orbits similar to ours--and aligned so that Kepler can "see" them--is about one-half of 1 percent. Given the sample size, however, that still leaves room for hundreds of potential discoveries.
But it will take three-and-a-half years of around-the-clock observations to capture the repeated cycles needed to confirm detection of an Earth-like world in Earth-like orbits.
Based on the first 43 days of data, scientists said in June they had identified more than 700 candidate planets and five possible multiplanet solar system.
The two Saturn-class planets announced Thursday, designated Kepler-9b and Kepler-9c, are part of a sixth system. Both planets orbit their parent star, 2,300 light-years from Earth, in gravitational resonance, with the former completing one "year" in about 19 days and the latter in about 38 days.
The Kepler observations are so precise, scientists were able to measure subtle timing variations that were the result of gravitational interactions between the orbiting planets.
"The discovery of the Kepler-9 system is important because it demonstrates an additional technique, this transit timing variations technique, that can be used for confirming planets and estimating their masses," said Matthew Holman of the Harvard-Smithsonian Center for Astrophysics. "This technique is something we will apply to future Kepler data where it'll be particularly valuable for estimating the masses of small planets."
For context, he said, "it's important to point out that if we were to observe the (Earth's) solar system from a distant star viewed edge on...we would see sizable transit timing variation. It gives you a sense of what Kepler's looking for. However, we can also use this technique to search for additional planets that may not transit the star. We can see their influence on the transit times of the planets that do transit the star."
Both of the Saturn-class Kepler-9 worlds are believed to have formed much farther from their parent star, "migrating" inward due to interactions with gas and dust in the original disk of material that gave birth to the worlds.
"Such a location so close to the star would not have been an easy place to form," said Alycia Weinberger, an astronomer with the Carnegie Institution. "So these planets probably had to form farther from the star and migrate inward and the fact that they wound up in this special relationship, this resonance, has something to say about the history of that migration. The whole history of that planetary system's formation may be included just in the present day configuration of these planets.
"It will take continued precise measurements by Kepler of their orbits and theoretical work to understand what the initial conditions were that lead to the final configuration that we observe today," she said. "But we could hope to understand how far these planets formed originally from their star and...how long it took them to form and how long their migrations lasted. And that really teaches us about the general mechanisms (of) the formation and migration of planets."
The smaller candidate planet, Kepler-9c, is believed to have a radius just 1.5 times greater than Earth's, completing an orbit every 1.6 days.
Earlier this week, a team of European astronomers announced the discovery of a solar system with up to seven planets, including five Neptune-class worlds, one possible Saturn-class planet and one possible low-mass world similar in size to Kepler-9c.
The European team used a ground-based telescope and a sensitive spectrograph to measure the slight movement of the parent star caused by the gravitational tugs of the orbiting planets over the past six years.
According to the Extrasolar Planets Encyclopaedia maintained by the Paris Observatory, 488 planets have been discovered to date beyond Earth's solar system, not counting the latest from Kepler. The radial velocity technique is responsible for the lion's share of the discoveries, but the transit technique will continue to gain ground as the Kepler data are analyzed.
"The Kepler data is like a treasure chest packed with all sorts of scientific riches," said Jon Morse, director of the astrophysics division at NASA headquarters. "Researchers across the country and around the world are just now beginning to dig through this treasure trove."