Role Reversal: Planet Controls a Star
Does this mean that the planet is not locked to the star but continues to rotate and not show only one side? If so, how is this possible? If not, and the planet is locked wouldn’t they both be locked by one another?
I feel like some important piece of information is missing…
They can’t directly measure whether the planet rotates or not. But as I understand it, if the larger body is tidally locked to the smaller one, that implies that the smaller one is tidally locked to the larger one. So the planet always shows the same side to the star, and the star always shows one side to the planet.
How is it that you understand that if the larger body is tidally locked to the smaller one, that implies that the smaller one is tidally locked to the larger one? I plead ignorance, but I could see how it could imply that they were mutually tidally locked at one time, but suppose the planet was impacted by a large mass that imparted rotation? In that case, it could just be the star that was tidally locked. And then I suppose that the rotation of the planet would eventually slow to a stop, but that could take a long time, no?
There’s a bit more here: Planet Forces its Star’s Rotation
It’s not obvious to me why they call it ‘tidal locking’ rather than ‘raising a tide’. Stars, the Sun at least, don’t rotate as solid bodies:
This paper says that astronomers assume that a close-in planet like Tau Bootes would be tidally locked to its star.
I haven’t seen any papers where anyone has calculated the actual time it would take for the planet to tidally lock to the star. But I’m pretty sure it would be a short time compared to the timescale of the lifetime of the system.
There’s another problem with the scenario you propose: there usually aren’t objects big enough for an impact to affect a planet’s rotation (especially a planet as big as Tau Bootes, which is at least 3 times as massive as Jupiter) once a solar system is formed. You need something the size of a planet to do that, and bodies that size with orbits crossing the orbits of other bodies in the system get collected into planets or ejected from the system pretty early in its history.
Oh, and this page has a crude estimate for how tidal synchronization time scales with mass. It has it scaling as the eighth power of the ratio of the radii of the two bodies. So if the star’s radius is (say) 10 times that of the planet (it’s almost certainly more than that), the planet will tidally synchronize to the star about 10^8 times faster than the star will to the planet.
Thanks for the good info Anne Neville.
Another partially related question:
What if the planet had a large moon? Couldn’t the moon’s orbit disrupt the tidal lock of the planet with the star?
That’s an example of the three-body problem. If I could solve that, I’d probably have a Nobel Prize.
The only cite dealing with this that I could find basically threw up their hands and said “who knows what would happen or how long it would take”.
There are also issues of how a moon that was large relative to a 3 Jupiter-mass planet could form. We’re talking probably something at least 1% the mass of the planet (Earth’s Moon is about 1/81 the mass of Earth), which comes out to about 9 Earth masses. For comparison, the largest object that formed in orbit around the Sun is Jupiter, which is about 1/1000 the mass of the Sun. Jupiter’s largest moon, which we think formed in orbit around it like the planets did around the Sun, is of order 10^-5 the mass of Jupiter.
You’d probably need a scenario like the formation of Earth’s Moon to get a moon that large, and I’m not sure that would work with a gas giant like Tau Boo. For starters, you’d need a planet on an orbit that crossed Tau Boo’s that was of order 10% the mass of Tau Boo, or 90 Earth masses :eek: We think Jupiter’s core is only of order 10 Earth masses (after that, it started accreting hydrogen and other gases to form the rest of it). I’m not sure you could get such a big object on an orbit that crossed other planets’ orbits, and I’m also not sure if the giant-impact scenario would work to form a moon after hitting a gas giant.
The other way to form a moon is to capture it into orbit. If there were something that size in an orbit that crossed Tau Boo’s orbit, I’m not sure Tau Boo could capture it into orbit as a moon. Captured satellites in our solar system tend to be small- most of them are asteroids a few kilometers across, and the biggest is Neptune’s moon Triton, which is about 0.003 Earth masses. Triton is a bit of a special case- Neptune is near the Kuiper Belt, so there are more large objects flying around than there would be closer in to the star. And Neptune is about 5000 times the mass of Triton- I’m not sure the capture scenario works for a moon so large relative to the planet.
And then there’s the question of how the planet got so close to the star, and whether a moon could form that close in or survive whatever process made the planet move inward toward the star.
And yes, I know, Tau Boo is really the star, not the planet. But I used to study extrasolar planets, and we always referred to the planets this way 
Who cares about the boring old stars? 
Note: all of the above is based on an estimate of 3 Jupiter masses for Tau Boo. That’s a minimum mass. It’s more probably something like 7 or 8, which makes the problems worse.
Fascinating!
Thanks, that article did contain some sentences missing from the other one.
jawdirk, the planet in question orbits only 5 million miles from its sun and its “year” is 3.3 days. There’s no conceivable way to stick a moon into a situation like that. Even if you broke up the planet you wouldn’t get a stable third-party orbit.
The general three-body problem is a beast, but there are many special cases of it which are solvable. Like this one, for instance. Consider the Earth, Moon, and Sun. If we just had the Earth and the Sun, the Earth would eventually lock to the Sun (after a very, very long time). But the Moon would prevent that from happening, since the Earth will lock to the Moon before it has a chance to lock to the Sun. Now, granted, in our solar system, both locking timescales are too long to occur before the Sun dies, so this is not an issue here. But one could surely construct an example with different timescales where a large moon does prevent an object from locking.
And I don’t think that the differential rotation of the Sun is relevant, here. I used to do some work with close binary stars (where the stars are locked to each other), and I seem to recall that locked stars do rotate uniformly. This planetlocked star is surely a weird case, but I would expect it to behave more like a starlocked star than like our Sun.
This sounds like what would happen if the Earth had a wholly liquid surface.
I don’t buy it.
It is not unique for a star to have a rotational period of 3 days, it is not unique for a planet to orbit in 3 days.
I suppose if Mercury’s orbit was close enough to orbit every 25 days, they would suggest that Mercury would be bullying the rotation of Sol.
Peace through Liberty
rwjefferson