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Dr. Neal Sumerlin keeps us abreast of happenings in the night sky and the progress of the new Belk Astronomical Observatory.

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Other Earths

Posted on 05/20/2009

I remember very well when I was a kid reading every astronomy book available in our school library.  Our town was too small for a public library, but in the east Texas oil fields, one of the main employers was Pure Oil-an oxymoron if there ever was one.  Pure Oil didn't like paying taxes, but they wanted a good public school system for their employees' families, and the library was fairly well stocked.  One of the topics that fascinated me most had to do with how the planets of our solar system might have formed.

There were two main competing models, even though there were then serious problems with both of them.  In one, a passing star pulled material from our sun which then condensed into planets.  The fact that the sun and the planets are all of the same age speaks to a common origin, however, not some subsequent chance encounter.  And as we learned more about the distances between the stars, the unlikely nature of such a near-miss became more apparent.  To give you some sense of scale: if our sun were a grain of salt, the next nearest star would be over eight miles away.  I would say that the salt grain is safe in its isolation.

The other theory (actually several different ones that share basic characteristics) had the planets forming in a disk around the newly condensing sun.  No one was quite sure how that happened, and there are still details that we don't fully understand, but this is the currently accepted model of planetary formation.

The "passing star" hypothesis would make our planetary system unique, or at best very unusual.  The so-called "solar nebular" model, however, might be duplicated in other star systems.  There was really no way of deciding between the two when your elementary-school-aged author was reading about them.

But there really was.  If there is any trend in astronomy, it is one of forcing humility on the human race, of moving it from the center of creation to a (physically, at least) peripheral location.  Copernicus showed us that the Earth was not the center of the universe in 1543.  Harlow Shapley in 1920 demonstrated that our sun was not at the center of the universe, but was about halfway out from the center of the group of stars we call the Milky Way.  And Edwin Hubble in 1925-yes, that Hubble, the telescope guy-showed that even the Milky Way Galaxy was only one of billions of similar galaxies, vast "island universes" of billions of stars, silently flying apart due to the expansion of space-time.  We are not at the center.  There IS no center.  So why should we expect that the processes that formed our planets were unique to us?

In 1995, the first planet orbiting another star was found.  I have written before about the most widely-used detection technique, the radial velocity method.  A massive planet orbiting close in to its star will pull the star to and fro as it orbits; the two of them actually orbit a common center of mass.  As the star moves toward us, its light is blue-shifted, and as it moves away, the light is red-shifted.  This is how the vast majority of the well over 300 extra-solar planets have been found.

This is not the way to find another Earth.  The method works best with really big planets (super Jupiters) orbiting really close to their stars (much closer than Mercury orbits the sun).  These are not the sorts of places where we would find water oceans and Earth-like environs.  In fact, the method would have a hard time detecting any planets at all around our sun.

There are other methods as well.  One of the most exciting  is the transit method, where a planet passes in front of its star as viewed from Earth.  This will cause the apparent brightness of the star to dim slightly, and the amount of dimming can give us the size of the planet.  If we follow this up with the radial velocity method described above, we can determine the planet's mass.  Put the two together, and we have its density, a good measure of whether the planet is rocky like the Earth (high-density), or gaseous like Jupiter (low-density).

We can study the atmosphere of the transiting planet by analyzing the light from its star as it passes through.  The presence of water would indicate the potential for Earth-like life; the presence of oxygen-well, that would be absolutely stunning.  Oxygen is a chemically reactive molecule that easily combines with surface minerals and must constantly be renewed in our atmosphere.  The presence of oxygen in our atmosphere, to the tune of 21% of the total, is due entirely to the life that covers our planet.

We can also determine the planet's temperature.  When the planet passes behind its star, only the radiation from the star itself remains.  The difference between this and the total radiation when the planet is visible allows us to determine the planet's temperature.

All of these-planet detection, analysis of atmospheric components, temperature determination-all of these have been done for planets orbiting stars that are light years from Earth.

But if you've thought about this a bit, you have probably realized that we must be lined up exactly right to be able to see such a transit, and that most planetary systems won't be so fortuitously arranged.  The chance that Earth would be lined up just right for some extra-solar astronomer would be only 0.47%, about 1 chance in 215.

Large numbers to the rescue!  An unlikely detection becomes very likely if you look at enough stars.  The recently launched Kepler spacecraft (http://www.nasa.gov/mission_pages/kepler/main/index.html <http://www.nasa.gov/mission_pages/kepler/main/index.html> ) will look simultaneously at over 100,000 stars in a fixed field of view, measuring their brightness every 30 minutes, for the next 3-4 years.  Think of what that 1 in 215 probability means when applied to 100,000 stars.  Assume they were all identical to our solar system, with twins of Earth orbiting twins of our sun.  Kepler would discover 465 Earths.

It will happen soon-you'd best get ready for a fresh dose of humility.  And when that report of a planet about the size of Earth, with a temperature that would allow liquid water to exist, and with oxygen detected in the atmosphere-when that comes in, get ready to change how you think about life in the universe.

Blog and journal content is produced by an individual. All opinions are those of the individual writer and may not reflect those of Lynchburg College.