habitable zone 宜居带
(male professor) So we were talking about distant planets that might be capable of supporting life, of course having liquid water is the key. The planet has to be close enough to its star so all the water isn’t frozen and far enough away that it doesn’t vaporize. And that comfortable area for water around star is called, anyone?
(female student) The habitable zone
(professor) Right, but in addition to the planet’s location, we need to consider the type of star that could support such a planet. As I mentioned previously, our Sun has some special characteristics that help make Earth habitable.
(male student) Oh, right, the Sun’s a solitary star.It doesn’t have any companions, so the planets revolving around can maintain a stable orbit.
And it’s the right size.
(male student) Well, if the star’s too big, it’ll burn out before life has time to develop.
(female student) And the star ’ s internal stability is important, too. It has to achieve, uh, equilibrium, I think you said.
(professor) Right, we say that the Sun is at equilibrium. That simply means that there’s a balance between the pressure pushing outward and the pressure pushing inward. The outward pressure, that’s the light and heat coming from the hydrogen burning at its core; the pressure pushing inward is gravity. This equilibrium allows the star to burn constantly for billions of years. So, those are generally the types of stars we look for in searching for habitable planets. And after looking at thousands of stars, we ’ ve identified about thirty that seem to be suitable candidates.
red giant 红巨星
white dwarf 白矮星
But, recently, an astronomer asked a totally unexpected question: “Is it possible that a habitable planet could be orbiting a white dwarf star?”
(female student) Um, a white dwarf?
(professor) OK, simply put, a white dwarf is the end result when a low mass star, less than half the mass of our Sun, when that star has died, burned all its fuel. Once that happens, the star’s core first contracts, then because of the heat generated by this contraction, its outer shell expands, so the star becomes huge, what’s called a red giant.
(male student) But that would destroy the planets around it, wouldn’t it?
(professor) Well, certainly the ones closest to it. But the outer planets might remain. Anyway, eventually, the red giant sheds its outer layer of gas and the hot dense core that’s left is called a white dwarf.
(female student) But you said white dwarfs are dead. So how can they have any of the characteristics needed for planet to support life, you know, like being in equilibrium?
(professor) Actually, a white dwarf is in equilibrium. Its core’s so dense it’ll generate heat and light for billions of years, long enough for life to develop. So, it’s really not an implausible notion at all. Still, the conditions wouldn’t exactly be earth-like. I mean, there’d be much less heat than the Sun generates. So any habitable zone would have to be much closer in. So close, in fact that a year on the planet would be about as long as one day on Earth. And there wouldn’t be any seasons because the gravitational pull from the star would be so strong that the planet’s axis wouldn’t be tilted.
solar eclipse 日食
(male student) But didn’t you say the innermost planets would have been destroyed?
(professor) Yes, and that’s why no one’s really been looking for planets around the white dwarfs. However, about twenty years ago, planets were found orbiting a different kind of dead star, a pulsar. Now, a pulsar’s the remnant of an extremely massive star that exploded. But new planets could have formed from the gas and debris ejected in this explosion or maybe the remaining outer planets could have been kicked inward somehow, and if it can happen around the pulsar,why not around the white dwarf?
(male student) So, how would astronomers find these planets around the white dwarfs?
(professor) Possibly by looking for stars that suddenly grow dimmer for a period of time that means a planet is passing between us and a star. And that’s the advantage of looking for planets around the white dwarf. White dwarfs are so small that it would be much more obvious when a planet passes in front of one, because it would block so much of the white dwarf’s light. It’d be like a solar eclipse, um, when the Moon passes between Earth and the Sun.
If white dwarfs were as large as other stars, it’d take a powerful orbiting telescope to detect the slight dimming as a planet pass in front of it. But you could see a planet eclipsing a white dwarf from Earth using a less powerful telescope.