Q&A: Why we need a new definition of the word ‘planet’


Is it time for a new definition of “planet”?

UCLA astronomer and planetary scientist Jean-Luc Margot says the answer is yes.

The current definition was decreed by the International Astronomical Union just nine years ago. The problem is, it applies to only bodies within our solar system.

Margot proposes solving this problem with a new, simpler definition that would apply to the thousands of known worlds that orbit stars other than the sun. More of these so-called exoplanets are being found all the time thanks to NASA’s Kepler space telescope and other planet-hunting observatories.

“We have thousands of bodies that need classification right now,” he said.

Margot outlined his proposed criteria this week at a meeting of the American Astronomical Society in Washington. He discussed his proposal in an interview with The Times.

How does the International Astronomical Union define a planet now?

The IAU’s official definition of a planet is a celestial body that (a) orbits the sun, (b) is nearly round, and (c) has cleared the neighborhood around its orbit.

How is your new definition different?

The proposal I make also has three components. It states that a planet is a celestial body that (a) is in orbit around one or more stars or stellar remnants, (b) has sufficient mass to clear the neighborhood around its orbit and (c) has a mass below 13 Jupiter masses.

What does it means for a body to clear the neighborhood around its orbit?

It is the dominant body in a region of space. A planet either ejects the material nearby, accretes the material nearby or controls the material nearby.

I should add that a planet can never completely clear its orbit because of the influx of comets and asteroids that are continuously going to end up there.

Why can’t you just apply the rules for our planets to exoplanets?

The 2006 rules require specifically that the body must orbit the sun, so right there we are not able to apply this criterion to exoplanets.

Beyond that, the roundness criterion is problematic. The size at which an object becomes round spans a whole range of values depending on its temperature, interior strength and thermal evolution. None of these things are observable from Earth for exoplanets right now.

Does planethood depend at all on what a body is made of?

No. It could be made of green cheese. It is nice that the criterion applies regardless of what the body is made of because we are not in a position to measure what most of these exoplanets are made of.

Why is it important to know whether a body is a planet?

When we applied the criterion to the solar system, it very clearly separated the bodies into two groups with three orders of magnitude between them. That’s nature telling us something about how they formed or their history.

A benefit of classification is it forces us to reorganize our thoughts, observe distinctions and ask how that happens.

What are the benefits of your new definition?

There is an equation behind it, and that removes some of the ambiguities in the existing definition.

Also, when you write down the equation for clearing an orbit to a specific extent in a specific time frame, it turns out that it depends only on the mass of the star, the mass of planet and the orbital period of the planet. One of the main advantages of the proposed criterion is all those things can easily be measured by Earth- and space-based telescopes.

Is it even possible to detect a body around a distant star if it’s not big enough to be a planet?

Probably not at this time. If you look at the roughly 5,000 exoplanets that we’ve detected so far, all of them exceed the proposed criterion by a substantial amount.

But it is not inconceivable that we would be able to detect bodies like Pluto or Eris around other stars in the future with more advanced technology.

How did you decide that a body 13 times the mass of Jupiter should be the upper limit of planethood?

I followed a recommendation from an IAU working group that considered this issue a number of years ago. Their recommendation was that a good threshold was 13 times the mass of Jupiter, because that is the mass at which deuterium starts to fuse inside a body. This upper limit is designed to distinguish between planets and brown dwarfs.

You made your case to your colleagues at the American Astronomical Society meeting. What was the response?

There is broad agreement that the current definition, although it was the best that was possible in 2006, is lacking and somewhat vague.

In the exoplanet session Friday morning, the chair of the Division for Planetary Sciences, Jason Barnes of the University of Idaho, advocated strongly in favor of pushing the proposal with the IAU.

Why does the IAU get to decide?

The IAU is the professional organization of astronomers and planetary scientists that has, for the past close to 100 years, been the arbiter of nomenclature of celestial bodies. It makes sense to have an organization and principles and a process to about naming bodies. Otherwise it would be chaotic and there would be no consistency.

I have to ask: How does Pluto fare under this new definition?

Pluto’s status isn’t changed. Pluto is not a planet. It very clearly fails to clear its orbital zone, by this definition or the previous definition.

I love Pluto. It is an amazing and fascinating world that is worthy of study, and none of that is diminished because of its classification.

This interview has been edited for length and clarity.

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