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Why It Is Impossible to Pinpoint the 1,000th Exoplanet

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Credit: NASA/Ames/JPL-Caltech

The list of known exoplanets is growing so long, so fast, that it is becoming difficult to properly appreciate the new discoveries. For those of us who grew up when our solar system accounted for the only nine worlds known in the entire universe, how are we to grasp the fact that astronomers now discover that many new planets in an average month? That at least 100 exoplanets have been added to the roster of known worlds every year since 2010?

The answer, I fear, is that we can’t grasp it. We have reached exoplanet overload. What was once a momentous discovery—the scientists who discovered the first planet orbiting a sunlike star, in 1995, were recently projected as Nobel Prize contenders—is now a blip on the scientific radar. Unless a newfound planet is especially exotic, or compellingly Earth-like, most of us yawn, if we hear the news at all.

Some astronomers are therefore eager to celebrate a coming milestone that will highlight the combined significance of all the discoveries—everything that exoplanet researchers have accomplished since the field began in earnest in the early 1990s. Any day now, the tally of planets listed in the online Extrasolar Planets Encyclopedia will surpass 1,000. As of this writing, the count sits at 992.

Discovering so many planets in roughly two decades is a remarkable achievement, and it’s as good a time as any to celebrate the sizeable haul of distant worlds. But it’s also worth noting (as some astronomers have done) that trying to pinpoint any particular planet as number 1,000 is a fool’s errand, somewhat akin to singling out one baby as the six billionth person on Earth.

Of course, there are far fewer known planets than people on Earth, so the odds of success would seem better on the planetary side of things, but at least everyone agrees on what a person is. There is no such consensus when it comes to defining an exoplanet. If an object’s planetary nature depends on its formation history (that is, a planet is something that forms in orbit around a star), then what are we to make of the free-floating, planetlike objects that fill the galaxy, whose origins are somewhat mysterious? Alternately, if a planet is defined by mass—bigger than an asteroid but smaller than a brown dwarf—where do we draw that line? Any definition of “planet” must involve some degree of ambiguity or else impose arbitrary distinctions.

Moreover, the Extrasolar Planets Encyclopedia is an inclusive list by design, boasting a significantly larger tally than some other lists. The NASA Exoplanet Archive, for instance, today lists just 909 planets. The more selective catalogue at exoplanets.org stands at just 732 confirmed planets and may take a while longer to reach the 1,000 mark. On any such list, it is a near certainty that some of the claimed discoveries will evaporate under further scrutiny or turn out to be larger objects, such as stars or brown dwarfs, rather than true planets.

So perhaps it’s time to borrow some verbiage common to astronomy, a science in which imprecision and order-of-magnitude estimation are facts of life. An astronomer might couch the achievement in words that are true today and will be true for the foreseeable future, regardless of how all the messy details shake out: the number of known exoplanets is now on the order of 103.

The views expressed are those of the author and are not necessarily those of Scientific American.

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