The citizen science project Planet Hunters has detected an exoplanet that fell through the cracks of the automated computer searches.
NASA’s Kepler space telescope detects transiting exoplanets through a dip in a light curve of a star, but sifting through reams of data is a daunting task. Computer algorithms have been designed to search for transit signals, but the human eye is also adept at seeking out signs of planets.
The Planet Hunters program is part of the Zooniverse network, and since December 2010, more than 290,000 members of the public have inspected 22 million light curves, equating to 180 years worth of 40 hour working weeks.
While computers can pick out short period planets easily, the citizen scientists compliment this system by finding planets with longer periods. This is because people can recognise a single transit, where as a computer usually needs to combine several transits before realising that a planet might exist.
The Planet Hunters detected a planet orbiting the star PH3 (also called Kepler-289). PH3 already had two known planets; PH3-b orbits the star in 35 days, and PH3-d has a period of 125 days. The discovery was announced in a paper published in the November issue of the Astrophysical Journal.
The newly discovered planet sits in between the two other planets and has a period of 66 days. PH3-c eluded computer algorithms because of its unusual properties. The gravitational forces exerted on the middle planet by its neighbours cause its orbital period to change. This is known as transit timing variation (TTV), and it means that instead of occurring exactly every 66 days, the transit of planet c will be offset by around five hours.
TTVs are more commonly on the order of minutes, and indeed the outer planet of the system has an orbit that varies by several minutes. However, the five hour variation of planet c caused an extra wiggle in the light curve that made it difficult for the computers to detect the planet at all.
This extra wiggle in the data is good news. TTVs can be used to confirm that the dip in the light curve is actually caused by a planet rather than by another star. They also have another important benefit.
Exoplanet transits only reveal the radius of the planet, and an upper limit of the mass of the planet can only be determined by follow-up spectroscopy measurements. Unfortunately, many of the Kepler stars are too faint for spectroscopic measurements, leaving the mass of the planet a mystery.
As TTVs are caused by gravitational interactions, and are thus influenced by the planetary masses, they can be used to determine the planet’s mass. Once the mass and radius are known, the density can also be found. PH3-c turns out to have an extremely low density, meaning it must have an atmosphere made mostly of hydrogen and helium.
This low mass, low density planet joins a growing group of similar planets that have been discovered using TTV.