WASP-17b: Low density, low mass, and high eccentricity

[The Overlord]

Saw this linked the other night, since then Wiki has a nice page for it.

WASP-17b is an exoplanet orbiting the star WASP-17 whose discovery was announced on 11 August 2009.[2] It is the first planet discovered to have a retrograde orbit, meaning it orbits in a direction counter to the rotation of its host star.[2] In terms of diameter, WASP-17b may be the largest exoplanet discovered yet, and at half Jupiter's mass, this would give it the lowest known density.

http://en.wikipedia.org/wiki/WASP-17b

[Oniya]

Interesting.  It makes me wonder how low a density a planet could have and still have enough gravitation to hold together.

[The Overlord]

I would tend to think it's pushing toward the limit, but typically a planetary mass that forms in a primordial star system is going to tend to hold together with what it has until acted upon by extreme external forces. Such as collision with another large object or tidal forces from said object.

At perihelion this thing passes one-seventh the distance from its star as Mercury to the sun, which might be skimming through the solar corona. The tidal flexing from the star is swelling the planet to extreme size. If it’s not ripping mass off the planet directly, I'm guessing the solar wind is wreaking havoc on the cloudtops; at half the radius and twice the mass, Jupiter has a gravity of 2.528 g near the cloudtops, so the upper atmosphere of WASP-17b must be tenuous indeed.

[Vekseid]

Interesting.  It makes me wonder how low a density a planet could have and still have enough gravitation to hold together.

Zero. One of the first calc III equations we had to solve was a gas cloud of infinite size but finite mass...

Inside a star system, you would calculate a planet's roche lobe, which for a planet is basically going to be its hill sphere, roughly given by

r = R * (M₂ / (3 * M₁))^1/3

Setting WASP-17b's mass to 1, the parent star's mass is ~2,600

r = R * (1 / 7800)^1/3

Or a bit over .05 times the radius. Not surprising since the parent star's mass is only slightly more than our Sun's, anyway.

Semiminor axis being roughly .05 AU, that gives a hill sphere radius of approximately 375,000 kilometers, or about three times the planet's radius.

However, the planet is far larger than any possible stable orbit around it. It is probably losing mass fairly rapidly, in astronomical terms.

[The Overlord]

However, the planet is far larger than any possible stable orbit around it. It is probably losing mass fairly rapidly, in astronomical terms.

Would come as no real surprise.