Monday, September 23, 2013

Einstein’s Planet

Kepler-76b, also known as “Einstein’s planet”, is a hot-Jupiter with twice the mass of Jupiter and orbits its parent star every 1.54 days. Its diameter is 25 percent larger than Jupiter’s. Kepler-76b is tidally locked to its parent star, which means it always presents the same side towards its parent star. As a result, the dayside of Kepler-76b blazes with an estimated temperature as high as 2250 K.

Kepler-76b is the first hot-Jupiter detected by the BEER algorithm - which stands for the relativistic Beaming, Ellipsoidal and Refletion/emission algorithm. The discovery of Kepler-76b was subsequently confirmed by follow-on spectroscopic observations. The BEER algorithm looks for three small effects that occur simultaneously as a planet orbits its star. These three effects are in addition to the dimming of a star as a planet transits in front and the radial velocity wobbling of a star due to gravitational tugging by an orbiting planet.

 Figure 1: Artist’s impression of Kepler-76b transiting its parent star. Credit: David A. Aguilar (CfA)

Figure 2: Light curve of the Kepler-76 system. Phase zero is when the planet Kepler-76b is closest to the observer, while phase 0.5 is when its parent star is closest to the observer, assuming a circular orbit. The secondary eclipse is clearly visible at phase 0.5 of the plot and the green line presents the BEER model. (Faigler et al., 2013)

For Kepler-76b, the “beaming” effect, sometimes also called Doppler boosting, is a relativistic effect that occurs when the star brightens and dims as the planet pulls it towards and away from us. The “beaming” effect created by Kepler-76b has semi-amplitude measuring 15.6 ± 2.2 ppm. Tsevi Mazeh of Tel Aviv University, one of the authors of the paper, mentions: “this is the first time that this aspect of Einstein’s theory of relativity has been used to discover a planet”.

In addition to the “beaming” effect, the team involved in this discovery also looked for the “ellipsoidal” effect whereby the star gets stretched into a football shape by gravitational tides from the orbiting planet. The star appears brighter when viewed from the side due to more visible surface area. Correspondingly, when viewed end-on, the star appears fainter.  The “ellipsoidal” effect induced by Kepler-76b has semi-amplitude measuring 21.5 ± 1.7 ppm. The third effect observed by the BEER algorithm is simply starlight being reflected from Kepler-76b and thermal energy being emitted by the planet. The reflection/emission semi-amplitude of Kepler-76b is 56.0 ± 2.5 ppm.

Interestingly, the team also found that the hottest spot on Kepler-76b is not the planet’s substellar point. Instead, the hottest spot on Kepler-76b is displaced eastwards by about 10 degrees due to the presence of an equatorial super-rotating jet stream within the planet’s atmosphere. The eastward displacement of the hottest spot on the planet shows up as an inflated beaming modulation between phases 0 to 0.5, which is where the hottest spot on the planet is “in view” (Figure 2).

Faigler et al. (2013), “BEER analysis of Kepler and CoRoT light curves: I. Discovery of Kepler-76b: A hot Jupiter with evidence for superrotation”, arXiv:1304.6841[astro-ph.EP]