Tuesday, July 8, 2014

Super-Earth in a Polar Orbit

Figure 1: Artist’s depiction of “sunrise” on 55 Cancri e. Image credit: Ron Miller.

55 Cancri e is a transiting exoplanet orbiting the Sun-like star 55 Cancri A. The suffix “e” indicates 55 Cancri e is the 4th planet discovered around the star. 55 Cancri e has 8 times the mass and twice the radius of Earth, placing it in the super-Earth-mass regime. The planet circles its host star in an unusually close-in orbit, racing around once every 17 hours 41 minutes, at an average star-planet separation distance of only 2.3 million km. 55 Cancri e is so near to its host star that its dayside is incinerated to a temperature of well over 2000 K, hot enough to melt most metals.

Spectroscopic observations using the HARPS-N spectrograph were conducted during the transit of 55 Cancri e across its host star. The observations allow the angle between the spins of the planet’s orbit and the star’s rotation, also known as the sky-projected obliquity, to be measured via the Rossiter-McLaughlin (RM) effect. Basically, the RM effect is a spectroscopic phenomenon that can be observed when a planet passes in front of its host star. As a star rotates on its axis, one half of its visible hemisphere will be seen approaching the observer and the other half will be seen receding away. Light from the approaching side would appear blue-shifted and light from the receding side would appear red-shifted.

When a planet passes in front of the star, it sequentially blocks some of the blue-shifted and red-shifted light, or vice versa. If the planet is in front of the blue-shifted portion, the star’s apparent radial velocity will have a positive value (i.e. the star appears to be receding) and if the planet is in front of the red-shifted portion, the star’s apparent radial velocity will have a negative value (i.e. the star appears to be approaching). The way the planet blocks the star’s blue-shifted and red-shifted light can reveal its sky-projected obliquity.

Figure 2: The solid red line shows the best fit to the observed Rossiter-McLaughlin anomaly of 55 Cancri e. The residuals yield a dispersion of 0.28 m/s. Bourrier & Hebrard (2014).

Figure 3: Schematic showing the view of 55 Cancri e. During transit, 55 Cancri e (shown as a black disk) crosses mainly in front of the blue-shifted half of the stellar disk due to its high sky-projected obliquity of 72.4°. Bourrier & Hebrard (2014).

In the case for 55 Cancri e, the measured RM effect is consistent with a high sky-projected obliquity of 72.4° (+12.7° / -11.5°). This indicates that the planet is in a highly misaligned and nearly polar orbit around its host star. Besides 55 Cancri e, the other 4 known planets around 55 Cancri A are also likely to be highly misaligned with the star’s spin axis. The orbits of the 5 known planets around 55 Cancri A are expected to be coplanar with one another despite being highly misaligned with the star’s spin axis.

55 Cancri A joins Kepler-56 as the two stars known to have highly misaligned multi-planet systems. In most multi-planet systems, including our Solar System, the planets all have orbits that are more or less coplanar with the equatorial planes of their host stars. 55 Cancri A has a companion star, identified as 55 Cancri B, at a distance of 1065 AU. It has been shown that the gravitational influence of 55 Cancri B is sufficient to have altered the alignment of the planetary system around 55 Cancri A to what is presently observed.

Bourrier & Hebrard (2014), “Detecting the spin-orbit misalignment of the super-Earth 55 Cnc e”, arXiv:1406.6813 [astro-ph.EP]