With its journey to find Earth-like planets in our solar system over, NASA's Kepler space telescope will embark on a new mission this week: the search for free-playing planets in an attempt to uncover how common these orphan worlds are, also referred to as the K2 mission.
Orphan worlds - a term for planets without host stars - are very hard to find, but they do exist. Scientists believe that they are the result of an ejection from their solar system families due to gravitational dynamics, whereby they are lured away by tidal forces. However, due to the difficulty involved in finding them, researchers are still unsure of just how common these planets are.
In order to answer this question, scientists are going to use a technique known as microlensing, a phenomenon that occurs when the gravity of an unseen foreground, such as an orphan planet, magnifies the light from a distant background star.
"For a lensing event that's due to a just a single object, the most salient feature of the light curve is the time scale," said Calen Henderson, an astronomer with NASA's Jet Propulsion Laboratory in Pasadena, Calif. "In general, a shorter time scale is potentially indicative of a lower-mass lensing object."
A free-floating planet that causes a lensing event could take place over a time period of days to hours.
"If you're just monitoring one star, it's very, very rare to capture a microlensing event," Henderson said. "That would only happen once per individual star every maybe 300,000 years. It's a fishing expedition. Microlensing events are rare. They're unpredictable and so what microlensing surveys have done for decades is just try to play the numbers game," he said.
Although some variations in star brightness stem from stellar flares, others are caused by bodies in the foreground that alter the light of the background star.
"What we're going to try to do with K2 is look for those events that are due to a lensing system that is just a free-floating planet," Henderson said.
The K2 team plans to use ground-based surveys in combination with the Kepler space telescope in order to get two perspectives of the lensing events and thus calculate the mass of the objects causing the lensings.
"It's the first chance that we will have to actually measure the masses of these objects and the first chance to find out if the ones we find that are low-mass are (gravitationally) bound to a host star or not," Henderson concluded.
NASA's Kepler team hopes to shed light on these unique orphan worlds by the end of the K2 mission.
