As humanity looks for life amongst the stars, we are beginning to discover other planets outside of our own solar system. These new planets, known as extrasolar planets or exoplanets, orbit around stars that are not our sun. NASA's Kepler Mission has detected over 18,000 such additional planet candidates. In a recent panel, NASA Jet Propulsion Lab Investigation Scientist Dr. Scott Edgington spoke about the Kepler Mission, how Kepler works, and how planet candidates are confirmed as actual planets.
Edgington began by bringing up the controversial topic of Pluto, which was downgraded from a full planet to a dwarf planet back in 2006. "Over the years, we've discovered many objects like Pluto out there," Edgington said. "They have similar characteristics to Pluto. They have similar sizes, similar masses, similar composition. When you start discovering, 200 or 300 of these, you start asking questions like, 'what is this?'"
Edgington stated that these discoveries caused a dilemma, and this dilemma eventually led to a better definition of what it means to be a planet, thereby kicking Pluto to the curb. Edgington compared the process to what biologists do with their own classification system when discovering new species of animals. "The reason why you classify things is that you learn something about it. It provides a means of communication. So if I say 'dwarf planet,' the first thought in your head should be Pluto."
So now that scientists have determined and understand what is and is not a planet, when the Kepler Mission discovers a likely candidate, a determination can be made about the finding.
What is The Kepler Mission?
Kepler itself is a spacecraft launched by NASA in 2009 to discover Earth-like planets that orbit other stars. The spacecraft was named after German astronomer Johannes Kepler, who is best known for developing the laws of planetary motion. The Kepler satellite is designed to examine portions of our own galaxy and then transmit its findings to Earth, where the data is analyzed to find and confirm new planets.
Kepler looks at what Edgington refers to as a "postage-stamp sized" part of the sky. The spacecraft measures the variations of brightness of more than 100,000 stars every half hour. From this data, it uses something called the "transit method" to detect a planet. This is basically the study of how brightness decreases due to a planet crossing in front of the star, or "transiting the star."
However, determining what is and is not a planet is complex, and there are often false positives using this particular detection method. "Not every effect that you see which diminishes the light in front of star is going to be a planet," Edgington explained. "There could be variability in the star, or it could be that there are other stars in the field of view that tend to add noise to the signal." It's up to the NASA scientists to study the data from Kepler and figure out exactly what it is they are looking at.
This is also why a second method for detecting a planet is part of the Kepler mission: a radio velocity, or Doppler, method. "This is where you look at a spectrum and you see how the light is red-shifted or blue-shifted and that tells you something about the orbit of that planet," stated Edgington. "You get the velocity from this and then, from this velocity, you get mass." Using these two methods — one which gives size and the other which gives mass — you can figure out a planet's density. "Density is the first piece of information that you can use in learning something about a planet," Edgington said.
As of 2013, using these techniques, Kepler has discovered over 2,700 potential planet candidates with 114 of those confirmed as actual extrasolar planets. The full list of candidates and planets can be found on the NASA's Kepler mission website.
The Search for Habitable Planets
But discovering planets is not enough. Kepler's main goal is to discover potentially habitable Earth-like planets with liquid water. But how can Kepler do this? "Basically, if you look at any solar system, you have a star — some hot, some cold," Edgington explained. "Around the star, you ask yourself, 'where's the liquid water?' If it's close to the star, a lot of water will be in the form of a gas because it's too hot for it to be a liquid. Outside of this habitable zone, it's too cold, so you have a lot of ice. The hard task for any scientist is to find out where the liquid water is." He summed up, "basically you're at the mercy of how hot the sun is."
In 2011, NASA confirmed that Kepler had discovered its first planet that resides in a habitable zone. The planet is "roughly a few times Earth's size," according to Edgington. He also believes that there are a lot more out there, even in Kepler's small patch of sky.
The Future of The Kepler Mission
The Kepler Mission was originally intended to end after just 3.5 years, but the mission has been extended to 2016. Edgington, however, is certain Kepler can continue beyond that. "I would imagine that as long as Kepler holds out and continues to function, I believe that it could just keep going." He stated that at some point, the spacecraft's gyros would give out, but that could be well beyond 2016. As long as funding is available, Kepler can continue to search for extrasolar planets indefinitely.
And who knows? Maybe one of those planets will support us Earthlings someday.