Another bad science headline

Bad, usually meaning "not quite accurate", science headlines really bother me. Today's example is this BBC article proclaiming "Exoplanets are around every star, study suggests". Except it's not entirely true, is it?

If you read the Nature letter [1], you'll find that what the authors did was to use a small set of gravitational microlensing events associated with known exoplanets to derive estimates of the number of exoplanets across a range of masses (Figure 2 from the paper). Using this plot and applying a bit more math, the authors estimated the expected number of planets (of all masses) in the galaxy, and by dividing that number by the number of stars, they discovered that "on average every star has [~1.6] planets…in an orbital-distance range of 0.5–10 AU" (emphasis mine).

Note, however, that contrary to what the BBC article suggests, this paper does not actually provide observational evidence that every star has at least one planet. The phrase "on average" is critically important and should not have been omitted from the headline—the authors' claims are no where as definitive as "exoplanet(s) around every star", and I'm entirely confident that if we pointed a telescope in just the right direction, we would find a star in our galaxy with zero planets.

I know all of this sounds overly pedantic, but when reporting about science, accuracy is paramount. I strongly dislike the seemingly pervasive perception in popular science reporting that grand claims, even when not entirely supported by the study being reported on, are necessary to make discoveries sound interesting and/or exciting. Such claims, especially when made in headlines, are a great disservice to science, as they leave the audience with a highly distorted view of what has been accomplished and often put scientists in the awkward position of appearing to abandon great discoveries.

In my personal conversations, I often refer to this as the "'broccoli cures cancer' effect", wherein a single experiment finding that compound A, found only in trace quantities in broccoli, kills tumor cells in vitro generates the headline "Broccoli cures cancer!!"

Rant about bad science headlines aside, this is a really cool study. This, along with a previous study [2] identifying planets not gravitationally bound to a star, leads us to conclude that "planets…in our Galaxy thus seem to be the rule rather than the exception."

References

[1] A. Cassan et al. "One or more bound planets per Milky Way star from microlensing observations". Nature 481: 2012. pp. 167–169.

[2] T Sumi et al. "Unbound or distant planetary mass population detected by gravitational microlensing". Nature 473: 2011. pp. 349–352.