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Tatooine's Double-Sunset a Common Sight
anthemaniac writes "Thirty years ago, Luke Skywalker beheld something that scientists are just now realizing is likely quite common in the universe: double sunsets. Astronomers have long known that binary star systems are common. And models suggested that planets could form in these systems, even though there's a double-tug of gravity on the material that would have to form a planet. Observations from NASA's Spitzer telescope, show that binary systems are just as likely to be surrounded by planet-forming debris disks are are lone stars."
Planets may be common in binary systems, but what about planets that support life?
One of the reasons that Earth can support life is that the distance between the earth and the sun remains close to a constant for the duration of Earth's orbit around the sun, so the Earth receives a fairly constant amount of solar energy. This means, for example, that the temperature doesn't go down to -200 in the winter and up to +800 in the summer.
But in a binary system, I would imagine that orbits that provide a constant amount of solar energy in the Earth-normal range would be less likely to occur. (What would such an orbit look like when there are 2 suns?) Are there any astrophysicists out there that can comment on this?
Doug Moen
I have written a truly remarkable program which this sig is too small to contain.
There are two stable possibilities: where the two stars orbit each other fairly closely (ie, 0-4 AU from the article, IIRC), and planets then orbit the common center of gravity formed by these two stars...or where one star has a very distant orbit, which is so far that it doesn't disrupt planets close in to the bigger primary.
If the second binary star is in a medium-sized orbit (ie, somewhere between where Jupiter and Pluto are in our system), it seems to be the case that this disrupts the planet-forming disk of gas so much that no planets are likely to form.
If you want to see a full list of all known exoplanets, go here: http://exoplanets.org/planets.shtml
The column marked "a (AU)" is orbital radius, where 1 AU is the earth's distance from the sun.
"The human race's favorite method for being in control of the facts is to ignore them." -Celia Green
500 A.U. is more than 10 times the orbital radius of Pluto.
Remember the inverse square rule:
A companion star even 40 A.U. far out would be just an especially bright star. If it had the same luminosity as the Sun, it would appear 1/1600 as bright (.0625%).
The Tatooine scenario is still romantic fiction: Stars close enough to appear in the sky together as visible disks would probably be close enough that planets in orbit around them to have strange orbits.
Planets and stars technically orbit each other, strange as that may seem when you say it out loud. In the case of the earth and the sun, they both swing around a common center of gravity. Because of the huge difference in mass however, this center is still located within the body of the sun.
Picture a long board with an anvil at one end and a small paperweight at the other. If you were to find the balance point between the two, it would certainly a lot closer to (perhaps underneath) the anvil. That stars revolve around this balance point produces a noticeable wobbling effect for each planet that circles it. In fact, this is one way in which scientists can guess if a star has planets in its system: by looking for this wobble.
With regard to binary systems, it would of course (as other posters have noted) depend how far apart the stars are and how close the planet in question is to either of them. Picture a mobile for a decent analogy. If you get two stars close enough together and a planet far enough away, the planet would likely calmly orbit the center of gravity common to the two stars and the planet.
....like their Oort cloud.....
What Oort cloud? There is no such thing. It is a mathematical fiction that has never been observed, even with the most powerful telescopes. The reason that such a fiction even exists is because comets should long ago have evaporated into space, having a lifetime of a maximum of about 15,000 years. That of course contradicts the accepted religious evolutionary dogma of the billions and millions of years for the age of the Universe. Hence the proposed fiction of the Oort cloud. True science is observation and experimentation, not conjecturing even in mathematical terms, about things that don't exist. Doing such conjecturing belongs to religion, not science.
All theory is gray
The bigger question would be:
Is it possible to create an orbit around a binary system where a planet has a stable enough environment for harboring life?
/.Mattsson - My native language is not English, so please don't whine over linguistic errors. (That's lame anyway...)