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How Many Planets Are in Our Galaxy? Perhaps Billions Upon Billions...

By March 3, 2011

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...Or maybe not.

The Kepler observatory, orbiting high above the Earth has spent about the last two years peering into the galaxy looking for planets circling distant stars.

Analysis of the first year's data revealed 1,235 planet candidates, with 54 of them being in the so-called habitable zone. A more detailed analysis, with additional data sets is needed to determine whether these objects are, in fact, planets (so far only 15 of these objects are confirmed to be planets). But the initial indications are that Kepler is about to significantly bolster the number of discovered exoplanets.

Now that the initial estimates are in researchers are eager to determine how many more planets, especially "habitable" ones, are in the galaxy. Since Kepler only sees one-four-hundreth of the night sky, the actual total number of planets is significantly higher than what Kepler can see.

The number grows even more significantly when you consider that Kepler can't possibly detect all the worlds in its field of view. For instance, scientists note that if a civilization were 1,000 light-years from Earth and using similar technology they may catch a glimpse of Venus passing in front of our Sun. But the chances of them catching both Venus and Earth is about one in eight.

Based on these types of probabilities, researchers working on Kepler believe that about half the stars being observed have at least one planet around them (they probably have several). And about one in a hundred of those stars has a planet in the habitable zone.

The initial data set indicates this all seems reasonable. However, things get a little hairy when you begin to extrapolate the numbers out to the entire galaxy.

NASA is reporting that, based on Kepler's results, that there is likely 50 billion planets in our galaxy (one for every ~2 stars in our galaxy), and that as many as 500 million of them (one-onehundreth of that previous number) are potentially habitable. Could be. But I think those numbers are a bit on the high side.

This is because their analysis only works if all regions are the galaxy are created equal. And they assuredly are not.

Roughly the inner third of our galaxy is a dense, dynamic region known as the galactic bulge. Observational evidence suggests that this area of the galaxy is rife with supernovae, black holes, neutron stars and regions of massive star forming clouds.

Then of course there are the Milky Way's spiral arms. These tentacles of gas, dust and stars are quite the chaotic mess as well.

My point? With the potential for disturbances and collisions that exist in the bulge and arms, it would be much more difficult for planets to form. For the perils of planet formation look no further than the asteroid belt or Pluto's orbit for what can happen when a forming world is disturbed.

We live in a quite region of the galaxy that seems fit for the formation of worlds, but extrapolating this to the rest of the galaxy does not seem logical. Perhaps I am overestimating the effects of the bulge and spiral arms, but I don't think I am.

Personally, I would place the number of planets closer to 5 billion, with perhaps 50 million or less in the magical habitable zone. Still large numbers to be sure, but only about 10% of what the current published results would suggest.

In either case, there are still lots of places to explore and lots of potential places to search for life.


March 4, 2011 at 4:56 pm
(1) G Dub says:

Extrapolate even those lesser estimates out to the 100-500 Billion estimated galaxies in the Universe, and pretty soon you’re talking about some pretty big numbers of planets – assuming that our galaxy is somewhere around the norm for planet density. We end up with numbers that make 1 in a billion or even 1 in a trillion odds of an event occurring on one of them become commonplace occurrences throughout the system.

November 5, 2011 at 5:45 pm
(2) Santos says:

So… probabilities of similar life… is obvious that there are other living organisms in other parts of universe ;)

April 26, 2012 at 7:50 am
(3) Justin says:

There are actually twentsixtillion planets out there that number has 21 0′s so you can imagine how many planets there really is.

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