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View Full Version : Discussion: Planets Can Survive a Red Giant



Fraser
2005-Jun-29, 06:12 PM
SUMMARY: Our Sun is in the middle age of life, and that's a good thing for us here on Earth. But in a few billion years, when the Sun runs out of hydrogen to fuel its massive fusion furnace, it will balloon into a massive red giant, engulfing the inner planets, including the Earth, before it shrinks again into a white dwarf. Is that the end of our solar system? Maybe not. Although they might get a little (okay... a lot) scorched, the outer planets might actually survive the experience in one piece. German researchers have found the first planet orbiting a white dwarf star, so there appears there's a future for planets when their star becomes a red giant.

View full article (http://www.universetoday.com/am/publish/planets_survive_red_giant.html)

What do you think about this story? Post your comments below.

TuTone
2005-Jun-29, 08:02 PM
Has Scientist ever seen a sun turn into a white dwarf or is that their theory that the sun expands & then shrinks into a white dwarf after it loses its hydrogen?

What's a brown dwarf?

lswinford
2005-Jun-29, 09:11 PM
TuTone, a brown dwarf is essentially a star that failed. It couldn't sustain its fusion fires. There was talk a few decades ago when it was discovered that Jupiter appeared to emit more energy than it received from the sun that Jupiter then was a brown dwarf. I think they've tightened the rules of description since then, disqualifying Arthur C. Clark's erstwhile star (calling it Lucifer in 2010).

Actually, was needing a score card on this story. White dwarf orbiting a red giant and the large planet was orbing the red giant at about 9 million miles out--but was there one around the white dwarf too or was I misreading something here?

Also, wouldn't a planet's orbit decline, perhaps rather I should say descend, when the star loses mass in an eruption at the stage changes? I would think then that the planet behaves like a floating dock on a sinking lake, sinking to a lower equilibrium point--either that or fly off (in this case then to be captured by the companion star).

Don Alexander
2005-Jun-29, 11:22 PM
Hello from Jena (or near it)...

A few minor corrections:

- The constellation's name is Eridanus, not Erinadus.
- It's Mugrauer, not Magrauer, and Neuhäuser/Neuhaeuser, not Neuhauser.
- The La Silla Observatory is in Chile, just the team that found the planet is from Switzerland (Didier Quloz et al.).

Concerning the former posts:

Brown Dwarfs are balls of hydrogen and helium like stars that are not massive enough (less than 80 Jupiter masses, more than 13 Jupiter masses) to create the pressure and temperature to ignite hydrogen fusion. But they do ignite deuterium (heavy hydrogen) fusion (which works in the mass range 13 - 80 Mj), but since this is a rather rare element, it burns up quickly. After that they just fade.

The Gliese 86 system is made up of a 0.7 solar mass star (the big bright object in the picture) and an 0.55 solar mass white dwarf (the little spot). The planet orbits the star, not the white dwarf. The white dwarf was initially a star that was quite a bit more massive than the exoplanet-hosting star, so it evolved faster, became a red giant, a planetary nebula and then a white dwarf. This must have happened quite a while ago, because the white dwarf is already very cool. The interesting thing about this system is:

a ) It was initially a rather tight binary star system, yet it formed at least one planet.
b ) The planet is in orbit around what was initially the smaller star.
c ) The pelnet is still there after the more massive star turned into a red giant and then a white dwarf.

Concerning Jupiter: As I wrote above, the lower mass for deuterium fusion is 13 Jupiter masses. Jupiter, weighing - of course - 1 Jupiter mass, is thus very far from a "failed star". But even after 4.5 billion years, it it still emitting energy from contraction. The monoliths in 2010 form a "supercrust" around the planet which is impermeable, and then compress it to a pressure and temperature to achieve fusion. The problem is that Jupiter is much too light to achieve hydrostatic equilibrium - the radiation and thermal pressure of the fusion reaction would immediately blow the planet apart in a gigantic thermonuclear explosion. The only possibility would be for the monoliths to become transparent, letting radiation but not matter escape.

John L
2005-Jun-30, 03:51 PM
Yes, the planet does not orbit a red giant, but a regular star. The white dwarf is the core of a now gone red giant. I'm also very glad to see a planet in a know binary system. That opens up half the stars in the galaxy to exploration that had been thought to not be planetary candidates. Very cool news!

Planetwatcher
2005-Jun-30, 05:55 PM
Could it be that the exo-planet origionally was in orbit of the white dwarf when it was still in the main sequence?
But then got caught in the smaller red star's gravity well when the origional star went white dwarf.

I'm kinda thinking that could happen in a tight binary system where a red giant woulld have enough gravity to keep the planet in it's grip, until it lost most of it's mass, and hence it's gravity influence.

If the planet was at a far point in orbiting the red giant turned white dwarf, but close point to the red dwarf which was before too small to influence the planet,
is it possible that the red dwarf was then able to grab the planet with the parent star's gravity influence now a fraction of what it had been?

I'm sure such an orbital shift could cause havak on the planet, but we don't know that didn't occur.

Nereid
2005-Jul-01, 12:08 PM
Originally posted by Planetwatcher@Jun 30 2005, 05:55 PM
Could it be that the exo-planet origionally was in orbit of the white dwarf when it was still in the main sequence?
But then got caught in the smaller red star's gravity well when the origional star went white dwarf.

I'm kinda thinking that could happen in a tight binary system where a red giant woulld have enough gravity to keep the planet in it's grip, until it lost most of it's mass, and hence it's gravity influence.

If the planet was at a far point in orbiting the red giant turned white dwarf, but close point to the red dwarf which was before too small to influence the planet,
is it possible that the red dwarf was then able to grab the planet with the parent star's gravity influence now a fraction of what it had been?

I'm sure such an orbital shift could cause havak on the planet, but we don't know that didn't occur.
I recall reading some simulations of planetary orbits in binaries, and the kind of thing you describe was not observed (I don't remember where I read it). It's quite likely that simulations have not explored all possibilities, especially the complex events towards the end of the red giant phase and the formation of the planetary nebula, so future work may bring a surprise or two.

However, I think it unlikely that the specific orbit change scenario you outline could happen. For example, the time it takes to form a white dwarf, from a red giant, is quite long, in terms of the likely orbital period of the planet.

Planetwatcher
2005-Jul-01, 06:06 PM
So then it's not like the red giant collaspes into a white dwarf over a series of days or weeks. but a much longer frame of time, which of coarse would not likely do the orbital shifts I asked about.

dragonmaster_us@hotmail.com
2005-Jul-01, 11:10 PM
But, then again, If you can imagine it, it has most likely happened at least once in the vastness of infinity.

Nick4
2005-Jul-26, 03:37 AM
Good thing that its not happening for another few billion years.