Friday, January 13, 2012

160,000,000,000 Milky Way Planets? 01.13.11

Have you ever played the party game where you try to guess how many candies were in a jar? You would look at the size of the candies, the size of the container, and try to visualize how many could fit in the space. Now, put that on a galactic scale.

Astronomers use math, not guessing, when trying to determine the number of the planets in our galaxy. There is not much guessing to this at all, really. There is, however, a great deal of calculations and observations. For years, scientists have been trying to track down planets elsewhere in our galaxy  using the “transit method” and “radial velocity.” The transit method measures light from distant stars. If the readings from the planet show regular dips in light, that indicates an object passing in front of the star at regular intervals, which, to cause such a measurable dip, must mean a planet in orbit. The radial velocity method measures how much stars “wobble” because of the pull of the planets in orbit around them.

A new observation method is being employed that is not so biased towards close-orbiting planets, called gravitational  microlensing. According to, microlensing measures how light is magnified and bent by gravitational fields from distant bodies. Based on the researcher’s calculations, they estimate at least 1.6 planets per star in our galaxy, in orbits from their stars roughly the same orbital range of Venus to Saturn from our own sun.

Based on that estimate—1.6 planets per star—and their estimate of about 100 billion stars in the Milky Way, that comes out to quite a few planets. When we here at the    Wetherbee Planetarium start to think about how many galaxies are estimated to be in existence in our Universe—100 trillion—it is mind-boggling to try to comprehend how many planets may be in existence outside our tiny little terrestrial world.
An artist's impression of the prevalence of planets in our galaxy.
CREDIT: ESO/M. Kornmesser

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