Expert: Courtney Seligman - 3/25/2011

Question
Its me again, the one who asked all those questions about astronomy before (http://en.allexperts.com/q/Astronomy-1360/2011/3/Questions-2.htm). Since then a few more questions have popped into my head.

- I know a binary star system/ multiple star system is were two or more stars orbit around eachother. Is it possible for those star systems to contain planets.

- The Oort cloud is supposedly over a billion miles away from the sun. How could something so far away be affected by the suns gravity?

- According to a local news station, astronomers discovered a planet containing liquid water orbiting a red dwarf. This planet is three times the mass of Earth, although it is said to be to close to it's sun to harbour life. Shouldn't this mean that the water would have evapourated?

- What exactally is a brown dwarf? I have also heard of a yellow dwarf, do they exsist or is it just rubbish?

Thanks!
Jeremy

Answer
If the two stars are close together, you could have planets far away from them, which orbit them more or less as if they were a single star. Or, if the stars were far part, then they could have planets close to each of them, which would orbit their neighboring star, and be only slightly affected by the more distant one. Where you'd have problems is if the stars were fairly far apart, so that only huge orbits would see them as a close pair, but close enough that planets in orbit around either one of them would be strongly affected by the gravity of the other; in that case, it would be difficult to have stable planetary orbits.

Actually, Saturn is nearly a billion miles from the Sun, so it, Uranus, Neptune, Pluto and all the Kuiper Belt and Oort Cloud objects are that far away or more, and all of them can be affected by the Sun's gravity. Admittedly, as thinga get far away, the Sun's gravitational force on them gets weaker, but as long as they move more slowly, that's not a problem. Stable circular orbits require an average orbital velocity proportional to the inverse square root of the distance. (that's a result of Kepler's Third Law of Planetary Motion, the Harmonic Law.) So, for example, the Earth moves 30 km/sec in its orbit (on the average). A planet 9 times further away (which is close to the orbit of Saturn) would have to move only 10 km/sec to be "held on to" by the Sun's weaker gravity at that distance (which, of course, is the approximate speed that Saturn has). A planet 40 times further away (such as Pluto) would have to move about 6 1/2 times (the square root of 40) slower than the Earth, or a little less than 5 km/sec, which is what Pluto does.

Of course, as you go further and further out, stable orbital speeds get lower and lower; but that's an automatic result of the way the motions work. Consider a comet which starts off in the Oort cloud, 20 thousand times further from the Sun than the Earth, and falls toward the Sun. As it does, it picks up speed, because the Sun's gravity is pulling it forward, faster and faster. By the time it reaches the Sun, it would be moving nearly 300 km/sec, and zip around it in hardly any time at all. However, once it does so, and heads back toward interstellar space, the Sun's gravity (pulling backward on it) would slow it down, until by the time it got back to where it started off, it would only be moving a few inches per second (in this example, it would be about 200 thousand times further away from the Sun at aphelion than perihelion, and as a result, would move 200 thousand times slower than the 300 km/sec it moved at perihelion -- that's Kepler's Second Law of Planetary Motion, the Law of Areas). So it's quite possible for the Sun to hold onto things, as long as they're moving very slowly when they are very far away, which is what they are all doing.

The main problem with the Sun holding onto things at that distance is that as we "orbit" the galaxy, other stars can pass by us, and tug on the bodies far from the Sun, as well. If the objects are more than 10 to 20% of the way from here to the nearest stars, odds are that their orbits will be so changed by such encounters that they would eventually be torn away from the Solar System, and wander around the galaxy on their own. However, for objects 5 to 10% of the way to the nearest stars, such near passages are more likely to slightly change their orbits, but not tear them away. It is such changes that allow comets which are now in nearly circular orbits far from the Sun to have their orbits "randomized", and in an occasional case, end up with orbits which fall nearly right into the Sun -- which is where the comets we see tend to come from. (In fact, that's the basis of the theory of the Oort Cloud. You need some way to get "new" cometary orbits, without just accidentally capturing new comets; because the latter would give you hyperbolic orbits which we never observe.)

As far as a red dwarf's planet is concerned, since such stars are hundreds of times fainter than the Sun, planets can be very close to them and have only average temperatures. In fact, that's the point of looking for such planets. Being close to the star, they move around it pretty fast, and have a pretty strong gravitational interaction with it. Since such stars don't have much mass, a good-sized planet orbiting close to them could noticeably affect their motion, whereas a smaller planet further away from a normal star wouldn't produce as noticeable an affect on their motion, which is why we haven't found any Earth-type planets yet. It's just a lot harder to find them, than to find big things close to small stars, or really big things near larger stars.

A brown dwarf is a red dwarf which is so cool that almost all of its radiation is invisible infrared light. We should probably call them infrared dwarfs, but infrared isn't a color; so it was suggested that perhaps, infrared being a sort of darker than usual red, and brown being a dark red, that they could be called brown dwarfs, and the name stuck.

There are yellow dwarfs, but they aren't what you might think. Namely, the Sun is a yellow dwarf. By and large, if a star is really big compared to other stars of its color and temperature, it is called a giant. If it is really small compared to other stars of its color and temperature, it is called a dwarf. The majority of visible stars which are yellowish are stars like the Sun, and stars much bigger than the Sun. So Main Sequence stars like the Sun, being the smallest common yellow stars, are called yellow dwarfs. Now, there are much smaller stars than the Sun that are yellowish -- namely, "white dwarfs" that are cool enough to be yellow. But the term white dwarf, although originally referring only to small white-hot stars, became a generic term for a particular kind of dead star -- an electron-degenerate star -- before many "yellow" white dwarfs were found. So they are called yellow "white dwarfs", rather than yellow dwarfs; but yes, such stars do exist.

Sorry for the "quick and dirty" answers, but I have to go out for a while, and it will be too late to answer your question when I get back; so I hoped it would be better to give you a short answer for each question, than to make you wait.