Nature: Distant star moved by tides

[bystander]

Distant star moved by tides
Nature News | Ken Croswell | 2011 Aug 09

Giant alien planet pulls surface of its star up and down.

[i]WASP 18's planet has the mass of ten Jupiters, and a year lasting one Earth day. (Credit: C.Carreau/ESA)[/i]

---

---

Astronomers believe they have spotted a distant star's surface rising and falling in response to the gravity of an orbiting planet, just as the Moon tugs Earth's seas up and down.

It was known that the planet, which orbits the star WASP 18 in the constellation Phoenix, would induce huge tides in its star because the planet is ten times the mass of Jupiter, and lies so close to WASP 18 that it orbits it in less than a day.

The planet lies at a distance of 100 parsecs from the Sun, and was reported in 2009. It first betrayed its presence by passing between us and its own star, causing the star to dim. Astronomers then confirmed the planet's existence by detecting Doppler shifts in the light from WASP 18 as the planet's gravity pulled the star towards and away from Earth¹.

But the planet's orbit posed a puzzle. In planets that lie very close to their stars, the force of the tides is so great that it changes the orbit from an ellipse — as seen in the planets of the Solar System — to a circle. "For orbits that are close in, the tides will attempt to take the energy out of the noncircular part of the orbit and dump it into the star or the planet as heat," explains Phil Arras, an astronomer at the University of Virginia in Charlottesville.

Yet the Doppler data suggested that the planet orbiting WASP 18 had a slightly elliptical orbit, similar to that of Neptune.
Gravity shift

Arras and his colleagues think that they have now explained the anomaly². The planet's orbit is circular after all, they say. The Doppler shifts in starlight that make the orbit seem elliptical are, in fact, caused by the star's surface rising and falling with the pull of its planet's tides.

When the star's surface rises toward us, its spectrum shifts slightly towards the blue, and when it falls away from us, it shifts slightly towards the red. The findings have been submitted to the Monthly Notices of the Royal Astronomical Society and posted on the arXiv server².

"With it being such an extreme system, we knew that it was going to be a good test of tidal theory," says David Anderson, an astronomer at Keele University in Staffordshire, UK, who helped to find the planet but is not involved with the latest study. Anderson says his "gut feeling" is that the new explanation for his team's data is probably correct.

Of all known planets, the one around WASP 18 raises the greatest tides in its star. Arras' team calculates that the Doppler shift shows the star's surface rising and falling at roughly 30 metres per second, which current instruments can easily detect.

Last year, astronomers reported that planetary tides were causing the star HAT-P-7 to bulge, making its brightness change depending on which side of the star was being observed³. But if Arras' team is right, WASP 18 is the first time that astronomers have actually observed a star's surface rising and falling in response to planetary tides.

Anderson and Arras would like additional observations to confirm the tidal idea. Arras says that a detailed re-analysis of the data may reveal a signature that can distinguish tidal movements from elliptical orbits.

1. An orbital period of 0.94 days for the hot-Jupiter planet WASP-18b - C Hellier et al
   • Nature 460 1098 (27 Aug 2009) DOI: 10.1038/nature08245
2. The Radial Velocity Signature of Tides Raised in Stars Hosting Exoplanets - P Arras et al
   • arXiv.org > astro-ph > arXiv:1107.6005 > 29 Jul 2011
3. The Discovery of Ellipsoidal Variations in the Kepler Light Curve of HAT-P-7 - WF Welsh et al
   • Astrophysical Journal Letters 713(2) L145 (2010 Apr 20) DOI: 10.1088/2041-8205/713/2/L145
     arXiv.org > astro-ph > arXiv:1001.0413 > 03 Jan 2010 (v1), 05 Feb 2010 (v2)

[neufer]

The tides induced on WASP 18 by this planet will draw it closer and closer to WASP 18 until it is either torn to pieces (within James Jeans's "danger zone") or falls into the star itself. (Hopefully such a decrease in orbital period around WASP 18 can soon be observed.)

This death spiral will happen eventually to all satellites that orbit a body faster than that body, itself, rotates (e.g., Phobos around Mars).

However, our Moon, Amalthea & Mimas all orbit their planets more slowly than those planets rotate so they should be in no such danger and should rather be slowly moving away from their parents as they gain angular momentum
(despite what Sir James Jeans says about them below).

THROUGH SPACE & TIME (1934)
BY SIR JAMES JEANS

<<If the little moon can stretch the big earth [to make tides], it stands
to reason that the big earth must stretch the moon even more,
and the same must be true of all the planets and their moons.
We can never see our moon being stretched, because we can
never see it broadside on, but we can see the process
very clearly in the case of one of Jupiter's moons.
A telescope shews that [Amalthea] is so stretched out
that it looks more like an egg than our idea of what a moon
should be. In course of time this little moon will move in even
closer to Jupiter. The nearer it goes, the greater Jupiter's pull will
be, and the more stretched and egg-shaped the little moon will
become Jupiter is stretching it out more and more just as
though it were a piece of rubber or elastic.

Yet we know that no piece of elastic can stand being stretched
out indefinitely. It must snap in time and so must the little
moon. Calculation shews that the moon will at first snap into
two distinct pieces, and Jupiter will have one more moon than
now. But as these two new little moons will still be just about as
near to Jupiter as the old moon was, they too will be strained and
egg-shaped. In time they too must break up, and, as the process
continually repeats itself, the number of Jupiter's moons will
increase indefinitely.

We can say that Jupiter is surrounded by a sort of danger zone.
When a moon or other body approaches this danger zone, it
becomes egg-shaped; when it finally enters the danger zone, it
is broken up and if it stays within the danger zone for long
enough it will be broken up into a vast number of tiny moons.

This is not mere guess-work, but the result of precise mathematical
calculation. As soon as we know the gravitational pull
either of a planet or of any other object, we can map out its
danger zone. There are naturally different danger zones for
different substances ; a cloud of tenuous gas is in danger in regions
where a rigid steely solid can venture in perfect safety. Now such
calculations shew that the little egg-shaped moon of Jupiter is
very near indeed to its danger zone. One of the little moons of
Mars [i.e., Phobos] is also near although not so near to the danger zone of
Mars, as one of Saturn's moons [i.e., Mimas] to the danger zone of Saturn.

This latter danger zone is of very special interest, because the
millions of little moons which surround Saturn and form its
rings are already inside it. It looks as if at some time in the past
an ordinary moon had wandered inside the danger zone of
Saturn and had been broken up into the millions of tiny moons
which now form the rings. These rings are a standing proof that
the danger zones have a real existence, and caution other bodies
as to the fate awaiting them if they get caught by the gravitational
pull of bigger masses. I have already reminded you of
Mr Kipling's story of how the elephant got his trunk. I
have now tried to tell you the story of "How Saturn got its
Rings" perhaps not with Mr Kipling's grace and charm, but
at least I believe that my story is a "really-truly" story, and not
merely a "Just-So" story.

Our own earth, too, has its danger zone. So far the moon has
kept well outside it, but in time the earth and moon must draw
nearer together, and as they do so the moon will become more
and more egg-shaped, until it finally crosses the danger line and
begins to break up. It is only a matter of time until we have a
fringe of rings like Saturn.>>