The paper One or more bound planets per Milky Way star from microlensing observations, which appears in today’s Nature, settles the question of whether planetary systems are common.
The paper’s 42 authors (who constitute a truly global collaboration of astronomers) analysed gravitational microlensing data gathered in the period 2002 to 2007. Now, gravitational microlensing – the short-term brightening of a star that occurs when the star, the planet of an intervening star and one of our telescopes move into alignment – is rare: at any particular time, less than one star in a million will be subject to a microlensing event. So although gravitational microlensing is a well-established planetary detection technique, it is less productive than either the radial velocity method (which is used by experiments such as HARPS) or the transit photometry method (which is being used by the phenomenally successful Kepler Mission). Thus the present paper does not announce the sudden discovery of a large number of previously unknown exoplanets. Rather, by statistically analysing the number of events that were detected, the team was able to estimate how many exoplanets are likely to exist out there.
The bottom line is: stars are more likely than not to possess planets. Our Galaxy must be teeming with planets.
The Drake equation contains a term fp – the fraction of stars formed that will have planets. Once upon a time, not that long ago, some astronomers believed that planets were rare. Perhaps, they argued, fp was small. Now we know for certain that isn’t the case. We know for certain that we can’t look at this term as a solution to the Fermi paradox.
It seems to me that every astronomical advance is simply sharpening the paradox.