Due to their relative faintness compared to their parent stars, most known exoplanets have been discovered using indirect detection methods – that is, detecting the effects they have rather than observing them directly. There are numerous indirect methods that have proven useful in the detection of exoplanets and now yet another, which relies on Einstein’s special theory of relativity, has joined the list with the discovery of an exoplanet known as Kepler-76b.
Since its launch in March 2009, NASA’s Kepler spacecraft has identified over 2,700 potential exoplanets and confirmed the existence of 122 using the transit method. This method involves looking for a drop in the brightness of a star as a planet transits in front of it. Because this dimming is very slight, Kepler needs to be extremely sensitive, and it is this sensitivity that a team at Tel Aviv University and the Harvard-Smithsonian Center for Astrophysics (CfA) took advantage of to detect Kepler-76b in a different way.
First proposed in 2003 by Avi Loeb and Scott Gaudi, the new technique relies on the gravitational pull the planet exerts on the star as it orbits it. This pull causes three observable effects. Firstly, the star brightens as it is tugged towards us because the photons from the star pile up and the light gets focused in the direction of the star’s motion due to relativistic effects. (Conversely, when it is pulled away from us, the star dims.)
Secondly, the team looked for an increase in brightness that would occur when the orbiting planet was pulling the star to the side so that it stretched into a football shape. This increases the brightness of the star because there is more visible surface area.The third effect is the least obvious and involved observing the starlight reflected by the planet itself.
The algorithm used to identify Kepler-76b was developed by Professor Tsevi Mazeh and his student, Simchon Faigler, at Tel Aviv University, and is called the BEER (relativistic BEaming, Ellipsoidal, and Reflection/emission modulations) algorithm.
Once identified, the new planet was confirmed by team member David Latham of the CfA using radial velocity observations gathered by the TRES spectrograph at Whipple Observatory in Arizona, and by Lev Tal-Or (Tel Aviv University) using the SOPHIE spectrograph at the Haute-Provence Observatory in France. The radial velocity method relies on observing variations due to the Doppler effect as the star moves toward or away from Earth in response to the orbiting planet’s gravity. Additional confirmation was provided by the transit method when it was discovered the planet also transits its star.
Kepler-76b, which the team has dubbed “Einstein’s planet,” is a “hot Jupiter” that has a diameter about 25 percent larger and weighs twice as much as the familiar gas giant. It is located about 2,000 light-years from Earth orbiting a type F star in the constellation Cynus every 1.5 days. Since the same side is always facing the star, the planet has a temperature of about 3,600° F.
The team found that the hottest point on Kepler-76b isn’t actually the point closest to the star (the substellar point), but at a location offset by about 10,000 miles. This is something that has only been observed once before and is a strong indication that the planet has extremely fast jet streams that carry the heat around it.
Like all other methods, this latest detection technique has its pluses and minuses. On the downside, it can’t be used to detect Earth-size planets using current technology. However, it doesn’t require high-precision spectra like the radial velocity method, and doesn’t require the planet to cross the face of the star as seen from Earth like the transit method.
"Each planet-hunting technique has its strengths and weaknesses. And each novel technique we add to the arsenal allows us to probe planets in new regimes," said CfA's Avi Loeb.
The team’s discovery is detailed in a paper that has been accepted in The Astrophysical Journal.