Wednesday, January 31, 2007

Reader mail

It's a slow news day here (unless you want to read about how I spent hours yesterday copying numbers from one file to another, and I sincerely doubt you do), so let's dip into the ever-burgeoning mailbag for a question. It was sent in early January by "super nova" (and appears slightly edited here): The Solar System was formed from a solar nebula, which occurred after a star expanded into a red giant. Does this mean that if Earth was formed this way, and living beings were created this way, the very same conditions should be taking place in other galaxies, meaning that there may be other life forms in other galaxies?

First, let me correct one small factual error in the first sentence. The Solar System indeed formed from a nebula, probably one much like the Orion Nebula. But this nebula did not come (directly) from a red giant star. The Milky Way Galaxy is pretty efficient at recycling matter. When stars finish their lives, they often have only used less than 50% of their hydrogen, the main fuel for stars. Bigger stars are less efficient, perhaps only using 10% (or less!) of their fuel, while more efficient models like the sun use over half of their fuel. When a star dies, whether by a supernova explosion or by a red giant star puffing up into a planetary nebula, all the unused hydrogen is returned to the Milky Way Galaxy, but this gas is far too hot to make new stars right away. Our galaxy mixes the gas up with cooler gas, and the hot gas radiates away heat to cool, and eventually it gets down to the temperature of most of the other gas in the galaxy.

By a method we don't understand, this gas sometimes gets collected into large, dense blobs called "molecular clouds." Molecular clouds can only be seen with radio waves, and they are like giant refrigerators -- the temperature of the gas inside is only a about 10 or 20 degrees above absolute zero (-273 degrees Celsius, or -460 degrees Fahrenheit)! When the gas is this cold, gravity begins to draw it close together to make dense clumps; these dense clumps form new stars. And it is from such a clump that our sun was born.

Our reader is right to guess that this process is happening everywhere. We see thousands upon thousands of stars being born across our galaxy. On average, we probably get one or two new stars every year in the Milky Way! We also see this same process happening in other galaxies, all the way to the most distant galaxies we can see! Many, though maybe not all, of these new stars also will have planetary systems. Our best guess right now is that at least 5% of stars have planets around them. Given that there are over 10 billion stars in the Milky Way, this means that there are at least 500 million planetary systems in our galaxy. And there are millions upon millions of galaxies in the Universe!

The harder question is, how many of those planets are capable of supporting life? The answer is: we don't know. We have no clue. In our Solar System, one of the eight (or nine) planets certainly has life, maybe Mars does or did, too. So maybe you could guess that one out of 10 planets in our Galaxy can host life. But we don't know how many planets there are in other solar systems. We can only detect Jupiter-sized planets or larger; these probably can't have life. But their moons may be able to.

Many other solar systems have giant planets very close to their sun; ours doesn't. In some of the other Solar Systems, planets have very oval orbits, meaning sometimes they are very close to their parent star, sometimes they are far away. In our Solar System, the orbits are pretty close to circles. This is good for life, because our temperature doesn't change drastically. Imagine if the Earth got as close to the sun as Mercury and as far away as Jupiter. At some parts of our year, the rocks under our feet would melt! At other parts of the year, the oceans and parts of our atmosphere would freeze solid. That would be bad.

NASA is working on a space mission to count how many Earth-sized planets there are in Earth-sized orbits around other stars. The Kepler Mission is very ambitious, and we don't know what we will find. Other astronomers are working on possible ways to detect life on those planets, maybe by looking for ozone or chlorophyll or other compounds that life creates.

Maybe in 10 or 20 years we can start to have a better guess as to how many planets there are capable of harboring life. But for right now, we only know of one. Maybe there are billions, maybe just one. In the meantime, we can focus on making sure the one we know about stays a good place for things to live. There may be no where else to go!

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