I’ve heard it said that yellow stars like are sun are probably the best ones to have planets with life. What about the red giant stars-are they likely to have earth-type planets? Or are they too dim to have warm enough planets? What about dual stars-do they have planetary systems as well? Having two suns would be neat. Incidentally, are most planets likely to be gas giant types (like Jupiter and Saturn)? Or are earth-type planets reasonably common?
A red giant is a temporary phase in a star’s evolution, and a late phase at that. Red giants happen when a star burns up all of its available hydrogen fuel and start fusing helium (or something heavier). Stars don’t start out as red giants, and planets only form when the star is forming.
If a star with planets became a red giant, the planets would still be there (unless they got swallowed up in the star when it expanded). I suppose the red giant’s solar system could still have a habitable zone, but it wouldn’t be the same as the habitable zone before the star went red giant. The red giant phase probably wouldn’t last long enough for life to evolve on a newly habitable planet.
Yeah, it’s all fun and games until somebody gets flung off into the interstellar void, or somebody crashes into a sun 
There are basically two ways to have planets in stable orbits in a binary star system. One is to have them much closer to one star than the distance between the two stars (they’re basically orbiting one star or the other, not both). The other is to have them orbiting much further away from the two stars than the distance between the two stars.
If the planet is doing something cool like orbiting in a figure-eight around the two stars, the orbit tends to be unstable, and the planet either ends up flung into interstellar space or crashing into one of the stars.
We don’t know. All we can reasonably detect right now are planets with masses more like Jupiter or Saturn than like Earth, and we’re best at detecting them when they’re close to their star.
Interactive Extra-solar Planets Catalog –> 200 planets!
Earth’s mass is about 0.0031 Jupiters. The smallest extrasolar planet thus far discovered is about 0.0122 Jupiters, 4X earth mass. That’s at the raggedy rdge of current detection limits, so no one can yet say how common earth massed bodies may be.
When I played Sid Meier’s Alpha Centauri, I used to sometimes remark that it should be titled “Alpha Centauri: Orbital Instability” (Alpha Centauri is a three-star system).
Yes, I realize that I am a big astro geek…
However, it’s effectively a binary as far as orbital stability is concerned – Proxima is waythehellouthere from the other two (0.2 light years) and much less massive (about 0.1 solar mass, compared to A at 1.1 and B at 0.9).
This text suggests that it is possible to detect objects as small as large asteroids orbiting around pulsar stars. (This particular cite may not be entirely credible, but I stumbled across it anyway)
Yes, but a planet in the wrong orbit in a binary star system (the figure eight around the two stars that I mentioned earlier) could still have an unstable orbit.
Pulsar planets are very different from planets around a Main Sequence star, or even planets around a red giant. It’s much easier to detect small motions in a pulsar, because the pulsar is putting out such a very regular signal.
Of course, planets around a pulsar wouldn’t be habitable- supernovae within a few light-minutes of a planet aren’t very good for living things, and neutron stars don’t put out the kind of visible-light radiation that life needs (they also put out quite a bit of destructive radiation).
While we’re at it, red dwarfs might not be so great, either. They last plenty long enough (significantly longer than our Sun), but the habitable zone would be so close to the star that any planets would probably be tidally locked. This tends to result in one side being too hot and the other side being too cold, and you’d likely end up with all the water frozen out and unusable on the dark side. There are potential ways around this (you might make your “planet” a moon, tidally locked to its primary instead of the star, or a harmonic lock might allow you to have a very slow ecosystem), but it’s much more difficult than with a warm, cozy star like our Sun.
It’s a good thing red stars don’t have planets with a lot of life, because otherwise their inhabitants would infest the earth, what with their Powers and Abilities Greater than Mortal Men.
The chirpsithra seem to be happy with it. 
Ralph, there is no reason why a terraformed planet could not support life around a red giant star – there’s a massive “habitable zone” where planetary mean temperature would stay within the liquid-water range.
However, life could not evolve around a red-giant star. Why? Because a red giant is a special, short stage towards the end of a star’s evolutionary path. It occurs when the star leaves the main sequence, as a result of helium-burning (or other, even higher element fusion).
The star would have spent most of its “lifetime” as a main sequence star, with a habitable zone significantly closer to the center of the star. The planets in that habitable zone would now be cindered if not engulfed – the greatly increased insolation from the larger solar disk would have elevated their temperatures, driving off all volatiles.
On the other hand, the red giant’s lifespan as a red giant is on the order of a million years – nowhere enough time for a planet to terraform itself.
Some future civilization with greatly enhanced engineering capabilities could no doubt run such a planet through forced terraforming and seed it with Earth life. Of course, we’re expecting a terrestrial planet, or possibly the core of a gas giant Jovian planet, to be in the habitable zone in order to make this possible. Alternatively, a Galilean moon or a Titan, satellite of a gas giant now in the habitable zone, might be a good candidate for terraforming, if it’s large enough.
And as long as it’s far enough away from its parent body so as not to be rendered uninhabitable by either the radiation bath or volcanism caused by gravitational flexing.
From this summary of the Draco Tavern stories:The chirpsithra are skinny 11-foot arthropods that have colonized worlds around red-dwarf stars throughout the galaxy and now convey other intelligent species to visit alien civilizations, providing them translators to talk with locals and each other. [emphasis mine]The Chirps evolved around sub-Solar mass red dwarf stars, which are distinctly different and much more stable than red giants (which are F or G class stars that have fallen off the main sequence of the Hertzsprung-Russell Diagram. Not being an astronomer I’ll leave to the professionals on the board to delve into the details stellar development and lifestyle, but the environments around the two are seriously different; a red giant is going to have a relatively short (on the order of millions of years) lifespan characterized by tempestuous behavior, while red dwarfs are stolid and predictable but with vary close habitable zones, resulting in the tidal locking condition Chronos describes.
However, be wary of interpreting conditions for life based upon what we (that is, all Earthlife) would regard as habitable; life (albeit, not as we know it) concievably could exist in environments far different than those found on Earth. But for it to abiogenically develop in complex or self-aware form in the dramatically unstable environment around a red giant seems unlikely at best.
Stranger
When planets were first discovered orbiting pulsars, they presented a problem to astronomers. The problem was that the supernova that created the pulsar should have vaporized any planets orbiting the star. So what were these planets doing in a place they had no right to be in?
Well, someone recently showed that it might be possible for planets to condense in the environment around a neuron star.
If I might tag along and hijack a bit, what about life on moons orbiting gas giants?
Arrrghh!! Somebody PLEASE get these planets out of my HEAD!!!