Just a short 25 years ago, if you had asked astronomers and astrophysicists whether there were planets around other stars, the answer would have been, “probably, but we don’t know for sure.” Thanks to a number of new techniques and advanced equipment, we’ve now discovered thousands of stars within our own galaxy that have their own Solar System. Planets come in a huge diversity of sizes and masses, and are found at all sorts of orbital distances; there are planets larger than Jupiter that orbit their star in less than 48 hours, there are Solar Systems with up to five planets interior to where Mercury is to our Sun, and there are over 200 Earth-sized planets discovered around those stars so far, plus 21 rocky worlds in the habitable zones of their stars.

Almost all of this information came from NASA's Kepler mission, which has been the primary exoplanet-discovering tool at our disposal. Yesterday marked the transit of Mercury, where our Solar System’s innermost planet passed in front of the Sun’s disk, blocking its light for a short period of time. At the start of a transit, the star’s brightness drops by whatever portion of the star’s disk is covered, then increases again when the planet moves off. That apparent dip in the star’s brightness, as tiny as it is, provides us with the very method that Kepler uses to detect planets around stars other than our own. When a planetary system is perfectly aligned with a star, relative to our line-of-sight, we can observe this transit, and detect worlds around another star.

The Kepler spacecraft observed a field of view containing approximately 150,000 stars over a period of approximately four years, detecting more than 2,000 planets and with over 1,000 additional “likely planets” that are still awaiting confirmation. But that doesn’t mean that only 1%-2% of stars have planets around them; the likelihood of having a good planetary alignment with our line-of-sight is very low, and furthermore, we can only detect planets with orbital periods that are less than Kepler’s observing time, so nothing farther out than Mars is.

When we compare what we’ve seen with what we expect to be there from the things we cannot yet see, we find some incredible things:

• About 80% of star systems are expected to have planets around them,
• The vast majority of planets are three times the size of Earth or smaller, not gas giant worlds,
• And that it’s estimated that there are approximately 60 billion rocky, habitable-zone planets in our galaxy alone.

Since my childhood I have always hoped that we would be able to find other life species in outer space, ideally intelligent ones.  It seems absurd that we could be the only evolving species in the universe.  Nevertheless the odds of contacting other life forms remain exceedingly long, not least because of the distances involved.  

David Fuller's view

Since my childhood I have always hoped that we would be able to find other life species in outer space, ideally intelligent ones.  It seems absurd that we could be the only evolving species in the universe.  Nevertheless the odds of contacting other life forms remain exceedingly long, not least because of the distances involved.  

Please note: I will be away on Friday.