Distant Star Moved by Tides

Recently, the scientific Journal Nature published an interesting theory online. Astronomers believe that the surface area of a distant star is influenced by the gravity of a huge alien planet. Earth’s moon is responsible for the tides that we observe here on Earth and equally, so the astronomers argue, the surface of a star is influenced by the gravity of the planets that encircle it.

It has been suggested that the planet, which orbits the star WASP 18 in the constellation Phoenix, would induce huge tides in its star. It has ten times the mass of Jupiter (which in turn has the mass of about 318 earths) and is so close to the star that it orbits it in less than a day.

WASP 18 is situated about 100 parsecs (i.e. 3.26 light years or 31 trillion kilometres) away from our sun. The alien planet was first observed when scientist noticed that the star would dim periodically, hinting towards a giant object passing between us and WASP 18. The planet’s existence was then confirmed by detecting Doppler shifts in the light emitted from the star reaching us.

A Doppler shift is the change of the frequency of a wave (here light) due to the Doppler effect caused by a nearby object to the observed phenomenon (in this case the huge alien planet). This phenomenon is used to determine distances in the galaxy as well as measuring gravitational phenomena in distant stars and galaxies.

However, the planet’s orbit posed an enigma. Planets that lie so close to a star would change their orbit from an ellipse to a circle. But the Doppler shift observed hinted towards an elliptical orbit of the giant planet.

However, Arras et al. argued that the gravity of the planet causes huge tides on the star’s surface, causing the observed change in the light path. Thus, the planet actually has a circular orbit.

Last year, another group of astronomers reported that planetary tides were causing the star HAT-P-7 to bulge, causing its brightness to change depending on which side of the star was being observed. However, if Arras’ team is correct, WASP 18 is the first time that astronomers have actually observed a star’s surface rising and falling in response to the gravitation of an orbiting planet. Arras’ team calculated the speed of the tidal rise and fall with 30 meters per second. These are tsunamis of completely different ball parks than those seen on earth. However, the surface of a star is hot burning plasma and not water, with a temperature of thousands of degrees.

Arras, P., Burkart, J., Quataert, E. & Weinberg, N. N. Preprint at http://arxiv.org/abs/1107.6005 (2011)

Hellier, C. et al. Nature 460, 1098-1100 (2009).

Image reproduced from www.nature.com

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