Massive exoplanets would induce huge tides on their parent stars when they orbit them at a close distance. Astronomers now suggest that current instruments should be able to detect them. Actually, they might already have detected such tides on a distant star.

Located 325 light-years away in the constellation Phoenix, WASP-18b is about 10 times the mass of Jupiter, and it orbits its star in slightly less than a day. The planet was discovered in 2009, as it caused the star to dim, when passing between us and its star. The existence of the exoplanet was later confirmed by measurement of the Doppler shifts in the light of WASP-18.

Usually, when a planet orbits very close to its star, the orbit tends to become a circle rather than an ellipse, because of very strong tides. However, the Doppler shifts suggested that WASP-18b had a slightly elliptical orbit.

Phil Arras from University of Virginia and colleagues think that the planet’s orbit is actually circular. The Doppler shifts that make the orbit seem elliptical would be, in fact, caused by the star’s surface rising and falling with the pull of its planet’s tides: the same thing happens with the rise and fall of sea levels on Earth, which are due to the gravitational forces exerted by the Moon (and the Sun).

As the star’s surface rises towards us, its spectrum is slightly blueshifted (the frequency increases), and it is slightly redshifted (the frequency decreases) when it falls away from us.

According to the team’s calculations, the star’s surface would rise and fall at approximately 30 meters per second, which is already easily detectable. Of all the planets known so far, WASP-18b raises the greatest tides in its star. Additional observations would probably help confirm this theory, and may even help distinguish tidal movements from elliptical orbits, avoiding possible confusions.



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