A newly discovered planet seems to have a surprisingly powerful influence on its parent star, forcing the star to rotate at exactly the same rate as the planet orbits. The planet's day is also the same length, so the pair are fixed in a face-to-face whirl.
The puzzle is how this planet, called COROT-Exo-4b, could have so dominated the vastly larger star, which is bigger than our Sun.
The new planet was discovered by the European COROT satellite, launched in 2006, which searches for transits – the telltale dimming of stars caused by planets passing in front of them. From the dimming effect on the light of its star, the COROT team worked out that that Exo-4b is roughly the size of Jupiter.
It is fairly close to its parent star, taking only 9.2 days to orbit, but that still puts it further out than most "hot Jupiters". The planet seems too distant to have such a strong influence on the star, says astronomer Suzanne Aigrain of the University of Exeter, UK.
Aigrain presented the discovery yesterday at a meeting in St Andrews, Scotland.
The planet's gravity will raise tides in the fluid body of the star, which would very gradually synchronise the planet's orbit and the star's rotation – but not within the billion-year lifetime of the system. "It would take longer than the age of the universe," Aigrain told New Scientist.
Instead, the system might have started out that way. "It could be a coincidence, but I don't like explanations based on coincidence," says Aigrain. She suggests that magnetic fields might have helped to lock the system together, but stresses that it is pure speculation at this point.
During its mission, COROT should discover many more planets. Aigrain and her colleagues hope they will then have a better idea whether synchronised systems are common, and what causes the phenomenon.
By Polemos
Fri Jul 25 14:30:33 BST 2008
The planet may have formed much closer to the star and initially revolved around the star slower than the latter rotated. The tidal interaction of the two closely spaced celestial bodies was intensive enough to rapidly transfer the star's angular momentum to the planet's orbital momentum--the planet receded to a higher and synchronous orbit.By Mike Wilson
Fri Jul 25 20:03:01 BST 2008
Maybe it is artificial? Are we looking at some mega-engineering project, perhaps some kind of device to extract energy from the star?By John Andrew
Fri Jul 25 20:11:26 BST 2008
Indeed. Good point. One day we will have a large enough telescope to resolve planet sized objects directly. Imagine the surprise when we detect a Banks Orbital?By John Wilson
Fri Jul 25 16:53:22 BST 2008
Suppose this planet has a conducting core. It is plausible that the star has a magnetic field, since the Sun does. Now, when a conductor moves in a magnetic field, there is a drag force on it. This can be observed in any electric motor or generator. Consequently, as the star rotates, its field would rotate and accelerte the planet.By Anonymous
Sat Jul 26 01:43:28 BST 2008
Agreed. I also think Aigrain's suspicions are probably right: it can a easily be a result of magnetic field coupling. We already have plenty of examples of simulations confirming theories of how the initially rapid rotation of many young stars is braked by interaction with its protoplanetary disk. A planet forming within a density concentration (possibly CLOSER IN than where it is now, as you suggest) could have interacted and migrated outward as the primary braked. A billion or so years seems like plenty of time to accomplish this to where we now find the planet and the tidally-locked primary. Makes sense - and it might be testable too, by looking for evidence of a strong magnetic field that still may be linking the two. Auroral emmission, or some other anomolous wavelength from the planet-facing hemisphere of the primary could tell.By Polemos
Sat Jul 26 05:51:57 BST 2008
Gravitational field is inversely proportional to the square of the distance between the interacting bodies.By John Wilson
Sat Jul 26 20:37:51 BST 2008
I think you will find that tidal forces are not proportional to the inverse square, but rather to the inverse sixth. Though my memory could be better.By Polemos
Sat Jul 26 21:14:44 BST 2008
The tidal forces are inversely proportional to the SQUARE of the distance between the interacting bodies:By Polemos
Sat Jul 26 21:46:15 BST 2008
Both of us have been wrong.By Gareth
Fri Jul 25 17:57:03 BST 2008
Maybe it is not a planet. A small compact object with a much higher gravitational field such as a neutron star, white dwarf or small black hole, perhaps surrounded by a small accretion disk might yield the signature seen by COROT. If the object orbits at a sufficiently small distance from the star it may not be individually observable. Alternatively the parent star lost some of it's angular momentum early in it's history (perhaps by passing close to another star) lessening the amount of work the planet needed to do to induce phase locking. I suspect suggestion two is more likely.All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
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18:53 05 September 2008
