October 29, 2013
If you’ve been paying attention to space news lately, you’ve probably seen reports of the discovery of the one-thousandth exoplanet. And you might be asking yourself, what was the name of this planet? Who discovered it? And why does NASA say that there are only 919 planets discovered?
The reality is that counting exoplanet discoveries is a process open to a lot of interpretation.
Exoplanet detection techniques have improved by leaps and bounds since the mid-1990s, when the first few planets were discovered, but it’s still an exceedingly difficult process with a lot of possibility for error. What initially looks like a bona fide exoplanet discovery may later turn out to be a false alarm. And sometimes a possible discovery won't be confirmed with a high degree of confidence until some time later, when astronomers using more advanced instruments or different techniques can prove that the planet exists.
For example, the Kepler mission searched for planets by looking for the dip in starlight that happens when a planet moves in front of, and blocks the light of, its star. This method, while extraordinarily productive in Kepler's case, has its weaknesses - sometimes small, dim stars can orbit in front of brighter ones, mimicking that dimming effect. As a result, Kepler planets aren't considered "confirmed" until further observation using different techniques and instruments proves with a high degree of confidence that they do exist. Much of the ongoing work on the mountains of Kepler data is dedicated to figuring out how to predict and weed out these so-called "false positives."
Different online databases of exoplanet discoveries have different criteria for what they count as an exoplanet discovery, and some are more lenient than others. There's no "official" count as much as there are various interpretations of what the count should be. When astronomers discover a new planet, they have to explain how "confident" they are in their discovery - in other words, how likely it is that the discovery is indeed a real planet and not a false alarm. This level of confidence affects what exoplanet databases will or won't count the planet.
What’s more, scientists have different ideas of what constitutes an actual “planet.” The line between a very large planet and a very small star is still a fuzzy one that isn’t fully understood.
Thus, the exoplanet counts you'll see online vary based on the criteria of that particular database. NASA's Exoplanet Archive, the database that PlanetQuest uses for its exoplanet counts, requires that planets be documented in a published scientific paper that's been reviewed and approved by other astronomers. It's more strict than the other online exoplanets out there that have logged one thousand exoplanets.
As for which planet is the thousandth, and who discovered it? It's a question that's difficult to answer. Planet discoveries often come in batches, and different teams of astronomers will announce new discoveries at the same time. Previously discovered planets might in the future be proven to be false positives, while possible discoveries that have already been made may be confirmed in the future. Not to mention that one astronomer's exoplanet might be another's brown dwarf. So, really, it's too hard to tell.
Still, the milestone of one thousand discovered exoplanets is worth celebrating, even if the details are open to interpretation. For generations, astronomers assumed that exoplanets either didn't exist, or were beyond our ability to find.
But in the past 13 years, astronomers have discovered one thousand exoplanets and revealed the galaxy to be chock-full of other worlds, including, very likely, planets like our own. Like most other exoplanet stories, it's a great reminder of the exciting, groundbreaking times in which we live
Here's to the many thousand exoplanet discoveries yet to come.
-Written by Joshua Rodriguez