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Who's counting? (redux)
NASA exoplanet count reaches 1,000. Why'd it take so long?
posted by Joshua Rodriguez
January 14, 2014
Who's counting? (redux)

Want an idea of how fast exoplanet science has exploded in the past few years? Consider that, for basically the entire recorded history of astronomy, scientists knew about a grand total of zero exoplanets.

Then, in 1995, the first was discovered.

Ten years later, that number had grown to 183.

Now, less than 10 years after that, the number has grown to just over 1,000 -- a nearly fivefold increase.

Of course, exactly how many exoplanets have been discovered depends on who’s counting.

PlanetQuest uses the official NASA exoplanet count at the Exoplanet Archive (http://exoplanetarchive.ipac.caltech.edu/), which is maintained by the NASA Exoplanet Science Institute (NExScI).

If you’re exoplanet-savvy and looking to dig deeper into the data that scientists have collected, this is a great one-stop-shop. You can sort planets or filter them into various subsets based on criteria you pick, get super in-depth information on stars and planets, and see actual Kepler data like light curves. All at http://exoplanetarchive.ipac.caltech.edu/.

Why is NASA’s count behind?

If you’ve been paying attention to exoplanet news as of late, you might be wondering, wasn’t the 1,000th exoplanet discovered a few months ago? Why is NASA’s count only now reaching this number?

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."

infographicThe pace of exoplanet discoveries has exploded in recent years.

Different online databases of exoplanet discoveries have different criteria for what they count as an exoplanet discovery, and all sites use different criteria. 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, as well as how the planet is announced, 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 19 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!