Second Exoplanet with Retrograde Orbit Discovered

The exoplanet HAT-P-7b has been observed to have a very curious orbit. It either has a highly tilted orbit – passing almost over the poles of its parent star, HAT-P-7 –  or a retrograde orbit; that is, orbiting in the opposite direction of its parent star. Two teams of researchers, both using the Subaru Telescope in Japan, have published papers on the bizarre properties of this planet, the second exoplanet ever observed to have a retrograde orbit.

In our Solar System, the planets calmly rotate in the same direction as that of their parent star, in our case the Sun. This is called a prograde orbit, and the Earth has the most inclined orbit with regard to the equator of the Sun, of 7.15 degrees. The planet HAT-P-7b, however, has an orbit that is the opposite of the rotation of its parent star but in the same plane as the equator (effectively a 180 degree incline). This is called a retrograde orbit. It may also be the case that it is inclined to at least 86 degrees of the equator of its Sun, so as to have almost a polar orbit. The researchers have yet to determine the true rotation of the star HAT-P-7, and thus which scenario is true for the exoplanet.

“There is a large range of uncertainty because we have not measured the true angle between the orbit and the stellar equator. Instead we can only measure the angle that we see from our perspective on Earth,” said Winn in a MIT press release.

HAT-P-7b is about 1.4 times as wide and 1.8 times as massive as Jupiter, and lies approximately 1,000 light years from the Earth.

A Japanese collaboration led by Norio Narita of the National Astronomical Observatory of Japan, and a team led by MIT assistant professor of physics Joshua Winn both published papers detailing their studies of HAT-P-7b. These studies were published in the Publications of Astronomical Society of Japan Letters October 25, 2009 and the Astrophysical Journal Letters for October 1, 2009, respectively. The paper by the Japanese team is available for your perusal on Arxiv here.

Both research teams used the Subaru Telescope’s High Dispersion Spectrograph instrument to observe the star HAT-P-7. The spectrograph allowed the researchers to monitor the redshift or blueshift of light as the planet orbited the star. In planets with a prograde orbit, their transit in front of the star blocks the blue shifting of the light from the star first, then blocks the redshift of the light, making the star appear to move more that it actually is.

In the case of HAT-P-7b the effect was reversed – that is, the redshifted light appeared bluer, then the blueshifted light appeared redder, making it apparent that the orbit of the planet was not in the same direction of that of HAT-P-7. This effect is called the Rossiter-McLaughlin effect, illustrated below.

The Rossiter McLaughlin effect makes a star appear to have a greater radial velocity than it actually does because of a transiting planet. Image Credit: Nicholas Shanks, WikiMedia Commons
The Rossiter McLaughlin effect makes a star appear to have a greater radial velocity than it actually does because of a transiting planet. Image Credit: Nicholas Shanks, WikiMedia Commons

The odd orbit of HAT-P-7b could have been caused by a number of different factors, and theorists that model the formation of exoplanetary systems will not have to “go back to the drawing boards”. The general consensus is that planets form out of a large disk of material orbiting the star, and thus all orbit in the same direction as the disk out of which they formed.

Multiple planets could have formed in an unstable configuration around the star, and their proximity to each other could have caused a rather chaotic series of gravitational billiards to boot HAT-P-7b into its current orbit. Another explanation is the presence of a third object in the system, such as another massive planet or companion star, that is tilting the orbit of HAT-P-7b due to what’s known as the Kozai effect.

The announcement of the retrograde orbit of HAT-P-7b came only one day after the announcement on August 12th, 2009 that the planet WASP-17b orbits opposite its parent star. HAT-P-7b is also one of the first exoplanets to be studied by the Kepler mission, which studied the planet’s orbit over 10 days. Kepler will take further images of the star during its mission, and by observing the rotation of spots on the surface of the star, nail down the orbital direction, after which we’ll know whether HAT-P-7b is orbiting “backwards” or around the poles of the star.

Source: Subaru Telescope, MIT

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