A WARM APPETIZER: On the hunt for cool, low-mass planets, which may take years to identify, Kepler needed little time to find much more massive planets kept scorching hot by their host stars. Above, an artist's conception of one such "hot Jupiter." Image: NASA/JPL-Caltech/T. Pyle (SSC)
Astronomers have filled in more details in the picture of the Milky Way Galaxy, unveiling five previously unknown planets outside our solar system that were detected via early data from NASA's planet-seeking Kepler spacecraft. A team of researchers reported the planet findings Monday at a meeting of the American Astronomical Society and in a paper to be published online Thursday in Science.
Kepler launched in March to seek out worlds like our own—terrestrial planets orbiting sunlike stars at a temperate distance where liquid water could persist. And although that search will likely take years, the spacecraft was able to pick out five larger, hotter planets in its first 43 days of observations.
Trailing Earth in an orbit around the sun, Kepler monitors the brightness of about 150,000 stars, looking for periodic dimming that might be caused by a planet passing in front of its star. The spacecraft initially identified 175 possible planetary systems for follow-up observation from the ground, says Kepler co-investigator Natalie Batalha, a professor of physics and astronomy at San Jose State University.
With 85 nights of observations on mid-size telescopes across the globe and 11 nights on one of the powerful 10-meter Keck telescopes atop Mauna Kea in Hawaii, the Kepler team thoroughly checked out 50 of those possible planetary systems, Batalha says. From the ground, the researchers looked for Doppler shifts in the suspect stars' light spectra that would reveal a gravitational wobble induced by an orbiting planet. Many were false positives, such as gravitationally bound binary stars eclipsing one another, but the five previously unknown planets emerged. As for the other 125 candidates, follow-up work is ongoing. "A bottleneck right now is having enough telescope time to follow up on all these interesting candidates that we have," Batalha says.
The five planets in the new Kepler results are all giants, with masses ranging from about 25 to 670 Earths. (The least massive, Kepler 4b, is roughly 1.4 times the mass of Neptune.) They are scorching, too—each planet's host star is about as hot as the sun, but the Kepler planets' orbits hug their stars so tightly that they complete a revolution in less than five days. For comparison, Mercury, the innermost planet in the solar system, takes 88 days to circle the sun.
The short orbital periods of the newfound planets enabled their detection from the small data set—each planet passed its star several times in the 43-day observation window, dimming the starlight by a small fraction with each orbit. Observing multiple orbits of cooler planets with longer orbital periods will take much more time. To find a true Earth analogue orbiting a sunlike star at about the same distance separating Earth from the sun—in other words, an Earth-size world with an orbital period of about one year—the Kepler team estimates that it will need about three years of observations from the spacecraft.
To complicate matters further, a planet of roughly Earth size would dim its host star only slightly and would induce very little wobble in the star, making identification and follow-up confirmation more difficult.
Habitable Earth-size planets might turn up sooner around smaller, cooler stars in Kepler's field of view, where water could persist on closer-orbiting planets that would complete laps around their host stars more quickly. But even so, exoplanet enthusiasts shouldn't expect pay dirt too soon—follow-up observations of any exo-Earth will be tricky and time-consuming. "The smaller the planet is, the harder it is to confirm," Batalha says. "It takes more telescope time. We'll be very careful with those."