The Latest on Exoplanets
Artist’s impression of exoplanet Kepler-78b, an Earth-size rocky planet that orbits its star very closely every 8.5 hours, making it too hot to support life. After discovering and measuring its size using data from Kepler, IfA’s Andrew Howard and colleagues used the Keck Observatory on Mauna Kea to determine the planet’s mass. “When you have both the size and the mass of an object, you can calculate its density, and thereby determine what it is made of,” said Howard.
Astronomers from the IfA and the University of California, Berkeley now estimate that one in five stars like the Sun have planets about the size of Earth and a surface temperature conducive to life. This conclusion is based on a statistical analysis of all observations from NASA’s Kepler Space Telescope.
Though Kepler is now crippled, it nevertheless provided enough data to complete its mission objective: to determine how many of the 100 billion stars in our galaxy have potentially habitable planets. A habitable planet is defined as one that is approximately Earth’s size and that is the right temperature for liquid water. “What this means is, when you look up at the thousands of stars in the night sky, the nearest Sun-like star with an Earth-size planet in its habitable zone is probably only 12 light-years away and can be seen with the naked eye,” said Erik Petigura, who led the analysis of the Kepler data. Petigura is a UC Berkeley graduate student working at IfA for a year.
“It’s been nearly 20 years since the discovery of the first extrasolar planet around a normal star. Since then we have learned that most stars have planets of some size and that Earth-size planets are relatively common in close-in orbits that are too hot for life,” said IfA astronomer Andrew Howard. “With this result we’ve come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way galaxy.”
Petigura, Howard, and Geoffrey Marcy, a UC Berkeley professor of astronomy, also used spectra of the planet-hosting stars from the W. M. Keck Observatory on Mauna Kea.
“For NASA, this number—that every fifth star has a planet somewhat like Earth—is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are,” Howard said. “An abundance of planets orbiting nearby stars simplifies such follow-up missions.”
The team cautioned that Earth-size planets in Earth-size orbits are not necessarily hospitable to life, even if they orbit in the habitable zone of a star, where the temperature is not too hot and not too cold.
In another recent study, Howard, Marcy, and their colleagues provided hope that many such planets actually are rocky like Earth. They reported that one Earth-size planet discovered by Kepler is the same density as Earth and likely composed of rock and iron, like Earth. This planet, named Kepler-78b, is uninhabitable because it has a temperature of more than 3,000 degrees F (over 1,700 C). “This gives us some confidence that when we look out into the habitable zone, the planets Erik is describing may be Earth-size, rocky planets,” Howard said.
All of the potentially habitable planets found in their survey are around K stars, which are cooler and slightly smaller than the Sun, Petigura said. But the team’s analysis shows that the result for K stars can be extrapolated to G stars like the Sun.
“If the stars in the Kepler field are representative of stars in the solar neighborhood, … then the nearest (Earth-size) planet is expected to orbit a star that is less than 12 light-years from Earth and can be seen by the unaided eye,” the researchers wrote in their paper. “Future instrumentation to image and take spectra of these Earths need only observe a few dozen nearby stars to detect a sample of Earth-size planets residing in the habitable zones of their host stars.”