The New York Times has a story about yet another weird extrasolar planet, this one a gigantic fluffy ball of gas bigger than Jupiter, but less dense than water:
While gas giants like Jupiter and Saturn are made primarily of hydrogen and helium, they also possess rocky cores and crushing pressures within that squeeze the hydrogen and helium to higher densities. Jupiter’s average density is 133 percent greater that of water, while Saturn’s is 70 percent that of water. The density of HAT-1-P is one-quarter that of water.
Astrophysicists now have two problems to solve: how a planet that has almost no elements heavier than hydrogen and helium can form and how it stays hot.
“Our lack of understanding is worse than we thought it was,” said Alan P. Boss, a planetary theorist at the Carnegie Institution of Washington who was not involved in the research.
You know, this has got to be one of the biggest down sides to doing astronomy. In a universe that is effectively infinite, even wildly improbable things are bound to exist somewhere, and when you find them, you somehow need to account for them. And, of course, you can only see a small fraction of what’s out there, so the weird things you find are ridiculously over-represented in your sample, and everything gets all out of whack.
I’m really glad I’m not expected to explain this stuff. Because that would distract from the coolness of a giant wispy planet orbiting a distant star every 4.5 days…
“The planet is wider than Jupiter but with half the mass, it is less dense than cork. In a cosmic bathtub, it would float.”
So…isn’t this just nonsense? If you had a large enough sphere of water to float this thing in, wouldn’t it either a. diffuse into the water or b. form an atmosphere around the water? Neither conclusion would surprise any layman.
Instead you make people imagine a large box shaped volume of water, supported by nothing, with a ball of gas floating half in and half out of it. Of course this troubles peoples’ folk physics intuitions!
Layman speaking here…How do you weigh a planet? Is it a matter of how it reacts to the matter around it? Or is it determined by the light coming off it? ??
Anyone know?
z.
In regards to this planetary discovery, I foresee speculations running ramped throughout physics. Adding a smidgen to this frenzy, I’ll recklessly surmise that this gigantic fluffy ball is being held together by – none other than – Dark Matter.;)
In our solar system, the easiest way to weigh a planet is to watch its moons (if it has moons): knowing the period and radius of any one moon’s orbit, you can find the mass of the planet.
It’s mighty difficult to see the moons of an extrasolar planet. In that case, you have to watch how much the planet perturbs the motion of its star.
Saturn’s moon Prometheus has an estimated density of only 0.27 g/cm^3, the lowest of any object in our solar system.
It’s mighty difficult to see the moons of an extrasolar planet. In that case, you have to watch how much the planet perturbs the motion of its star.
Which is indeed how it’s done: careful study of the star’s spectrum shows a Doppler shift due to the star’s “orbit” about the star-planet center of mass (the center of mass will be deep inside the star, so it’s really more of a wobble); from this and an estimate of the star’s mass, you can get the planet’s mass. There’s usually an uncertainty factor due to not knowing the orientation of the planet’s orbit — except that in this case we know the orbit is almost edge-on to our line of sight, due to the fact that it actually eclipses the star.
(It’s also possible to get the mass of a planet independently if it gravitationally lenses a background star, but I think there’s only one case of this to date.)
You know, this has got to be one of the biggest down sides to doing astronomy. In a universe that is effectively infinite, even wildly improbable things are bound to exist somewhere, and when you find them, you somehow need to account for them. And, of course, you can only see a small fraction of what’s out there, so the weird things you find are ridiculously over-represented in your sample, and everything gets all out of whack.
Of course, to my mind this is what makes astronomy more interesting than physics! 😉 Always something new and unexpected to find, and new ways to test other people’s theories…
(Not sure why you think the weird things will be “over-represented in your sample,” unless “you sample” means “what gets reported as new and interesting in the general press”…)