The Kepler Space Telescope was launched on March 7, 2009, in order to hunt planets in a certain fraction of the sky. The spacecraft maintains constant orientation as it orbits the sun while it scans more than 150,000 stars to detect possible planets.
And with the Kepler spacecraft’s data of star Kepler 62, UW associate professor of astronomy Eric Agol discovered a potentially Earth-like planet entitled Kepler 62-f, 1,200 light-years away.
“The Kepler spacecraft is basically a huge digital camera, collecting photons of light from stars,” Agol said, “And if a planet is in front of a star, it’ll collect fewer photons, so I go through a dataset … plot differences … and may get some spikes when there’s a planet transit occurring.”
When a planet transits, or passes in front of the star from the vantage point of the spacecraft taking pictures, some of the light from the star is blocked. The dataset referenced is composed of Kepler’s measurements of star brightness, tracked across time. The spacecraft snaps a shot of the sky every six seconds, adds up the shots over 30 minute intervals, then transmits the data to Earth. Determining the brightness of stars versus time can pinpoint when there are periodic dips in brightness. And if a certain pattern emerges wherein certain stars are dimmed regularly by roughly the same amount, then it’s plausible a planet is orbiting a star and blocking light emission.
“I plotted this curve for a particular star, and it was already known planets orbit this star, and I found these three spikes,” Agol said. “The thing that made me realize it was a planet immediately was that they have spacing exactly the same.”
The dimming of Kepler 62’s light was 267.3 days apart, and there was only one gap in the data to contradict Agol’s conclusion. However, that gap was due to faulty transmission on the Kepler spacecraft’s part. The software had consequently de-emphasized the spikes that Agol later found and realized were transits.
“The software they have requires three transits,” Agol said. “So they missed the transit.”
Through similar transit measurements, a team of researchers and astronomers led by William Borucki, principal investigator of the Kepler Space Telescope, had already discovered several planets orbiting Kepler 62 in the Lyra constellation. Their discovery of Keplers 62-b, c, and d was being prepared for publication when Agol, in August 2012, alerted them to his own finding.
“I was trying out an algorithm last summer and discovered a fourth planet after I’d removed the three previous ones they’d found,” Agol said.
Temperature, luminosity, radius, mass, and energy of the star were inferred through taking a spectrum of the star’s emission. Combining those properties with orbital periods generated by the transit measurements, the researchers confirmed the distance of the orbit, as well as the amount of energy each planet absorbed.
“The planet I found, 62-f, is cooler than Earth, with less energy hitting it,” Agol said, “But it’s in a range of the greenhouse effect that could allow for liquid water on the planet.”
Kepler 62-f is also 1.4 times Earth’s size, and absorbs roughly half the amount of solar heat and radiation as Earth. Nearby, 62-e is 1.61 times Earth’s size, and receives 20 percent more heat and radiation than Earth as well. These two planets are the smallest outside our solar system that have been found in a habitable zone.
“The habitable zone is more of a general guide,” said David Catling, UW professor of astrobiology and earth and space sciences. “Does it have liquid water and atmosphere, the right combination of liquid zones?”
The surface temperature of a planet is set by three factors: proximity to a star; the greenhouse effect, in which atmosphere warms a planet; and the Albedo effect, or the reflectivity of the atmosphere.
“There’s a possibility of ocean water on it, which means it could be habitable,” Catling said.
“We need to figure out whether it is a solid planet, or like Neptune, which is mostly gas. What’s difficult is we’re speculating about something when we don’t even know its overall density.”
The planet’s mass is also currently unknown, but its size indicates a rocky planet, much like Earth, which promotes an even greater chance of life given the possibility of condensation and water collection.
“We don’t know a lot about this particular target,” said Victoria Meadows, director of the UW astrobiology program. “It’s a pretty good candidate to be a habitable planet, but to prove it will require a lot more observation.”
And given the distance of Kepler 62-f, a much larger telescope than the Kepler Space Telescope will be required. Also, to verify the mass and density of 62-f, more powerful instruments will be necessary.
“The Kepler mission is doing its work,” Catling said, “replacing the unknown with the known.”
Alternatively, looking at other prospective planets that are closer could provide some information relating to the Kepler planets. The Kepler spacecraft is mainly gathering statistics and discovering possible exoplanets (planets outside the solar system). It is merely a pathfinder, as it were.
“The exciting thing is this Kepler Spacecraft is only observing 1/400th of the sky,” Agol said, “so the whole rest of the sky is there to search. That was the long-term goal of NASA, to have a terrestrial planet finder, look at closest stars, and see light from planets.”
Reach reporter Garrett Black at firstname.lastname@example.org. Twitter: @garrettjblackKepler space telescope, Eric Agol, Kepler spacecraft, kepler mission <BR/>