Upping the Number of Habitable Worlds

Last week, the SETI Institute put out a study that claimed there could be as many as 300 million potentially habitable worlds in our galaxy. That’s a pretty good number, considering that nearly every star is likely to have (or have had) at least one planet (according to another study). What’s more, given the distribution of stars and planets in the Milky Way, some of those potentially habitable worlds could be fairly close to us. This isn’t just some “pie in the sky” guesstimate. Scientist Jeff Coughlin (of the SETI Institute) and a team of researchers used Kepler Space Telescope data as well as observation sets from the European Space Agency’s GAIA mission to come to their conclusions.

Estimating the Number of Such Worlds

So, if there ARE 300 million potentially habitable worlds “out there”, what might they be like? Anybody who reads science fiction probably has a good idea. Visiting other planets is a staple of that genre. For those of us who read SF, the existence of worlds with life on them is not a new idea. But, for science to work, there has to be observational data of those worlds. That’s to feed any theories about what these places might be like. To answer the question, and come up with their estimate of 300 million, the SETI team devised a plan.

First, they studied other worlds that are similar in size to Earth. They looked for probably rocky worlds orbiting stars similar to the Sun. (SO, this cuts out rogue planets, which I discussed in my last entry.) And, of course, they wanted planets in the “habitable zone”. That’s where liquid water could exist on the surface of planets orbiting within it. Add in other factors, like how much light hits the planet from its star, and that gives scientists a good set of parameters for estimating how many such worlds exist. But, it still doesn’t tell us what they’re like.

Finding Life on Habitable Worlds

Determining life on distant worlds is not something we can easily figure out from Earth. Well, astronomers can use special techniques to study the atmospheres of those worlds. There are some chemical signatures in atmospheres that would indicate the presence of life. And, of course, one could direct the radio telescopes of the Allen Telescope Array, for example, in their direction, to see if any intelligent civilizations are beaming out signals.

That’s one way beings “out there” could tell if our planet has life, by the way. We’ve been sending signals to space for around a hundred years, from radio and TV broadcasts. They are racing out away from us at the speed of light, and there ARE some potentially habitable worlds within that signal path. (It’s actually an expanding balloon of signals called the “radio sphere”.)

Another way might be to look at our atmosphere as our planet transits the Sun. A distant observer, with direct line of sight to see such an event, would see sunlight streaming through our atmosphere. The gases in our air blanket would absorb specific wavelengths of light. That creates “dropouts” in the solar spectrum. Each element and chemical compound has a specific “fingerprint” that could show up in the spectrum. Those that life emits would clue in a distant researcher that there’s life on our planet.

The Search is Afoot

That is, in fact, what researchers plan to do: study the light from the star as it passes through a nearby planetary atmosphere. If the fingerprints of chemicals related to biotic activity are there, then it’s a likely sign that life exists there. Of course, it’s painstaking work. It isn’t done overnight. It requires detailed observations and a LOT of analysis. That’s the way the science works and such observations are happening.

They will take a while, so while we wait for that first momentous discovery, we can still marvel at the fact that there are 300 million planets out there to study. Some of them just HAVE to have life. We can’t be the only ones in the galaxy looking out and wondering who else is out there. Or, to put it another way, in his book with Ann Druyan, the late Carl Sagan wrote, “The universe is a pretty big place. If it’s just us, it’s just us, seems like an awful waste of space.”

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