What is a planet? Jean-Luc Margot wants a new definition

When Pluto was demoted from full planet status in 2006, it was because the International Astronomical Union’s definition of a planet had changed. The change created an uproar. But, since the mid-1990s, scientists have discovered more than 5,000 exoplanets, or worlds orbiting distant stars. On Monday, July 22, 2024, EarthSky’s Deborah Byrd spoke LIVE with Jean-Luc Margot, a Belgian-born astronomer and UCLA professor. Margot and his team just published a new study, outlining their reasons for believing we need a new definition of “planet” and suggesting a possible new structure for defining the word. They will present their ideas to the IAU General Assembly – a global meeting of astronomers – beginning on August 6 in Cape Town, South Africa.

Deborah Byrd: You’re watching EarthSky. I’m speaking with Jean-Luc Margot, who is a Belgian-born astronomer and a UCLA professor. Jean-Luc and his team have just published a new paper in the Planetary Science Journal, and they’re suggesting a new scientific definition for the word planet. Jean-Luc, welcome.

Jean-Luc Margot: Thank you for having me.

Deborah Byrd: As I mentioned to you before we started, I’d like to go back a little bit and talk about the other planet definition that the International Astronomical Union set up in 2006. It was that planet definition that caused Pluto to lose its full planet status and created a huge uproar among people. Can you tell us what happened there in 2006?

Jean-Luc Margot: Sure. I was actually at the meeting in Prague in 2006. And so I have a pretty good recollection of what happened. I was at the meeting because I’m a planetary scientist. I study planets. And there were multiple sessions, symposia and joint discussions at the General Assembly in 2006, devoted specifically to planetary science. There were sessions about planetary science exploration missions. There were sessions about near-Earth objects.

And so I was there to share my results with planetary science colleagues. And during the general assembly, which is a short duration meeting, maybe 10 days or so, the International Astronomical Union revealed a proposed definition for planet. It was the first time that the term planet was actually officially defined.

Interestingly, the way in which the proposal was revealed to the science community is highly unusual for the scientific community. We like transparency. We like working in the open and having comments and feedback on our papers and our different ideas. But for whatever reason, at that meeting, the Planet Definition Committee kept their proposal secret and they disclosed it via an embargoed press release. So the reporters discovered the definition of a planet before the scientists did, which is again, highly unusual. And immediately upon reception by the planetary science community, we realized there were substantial problems with the proposal and it had to be fixed. It just wouldn’t have worked the way it was proposed. And in the course of this 10-day meeting, which was of course packed with all kinds of scientific sessions, people tried to improve on that definition. And the result is what we have today. I would say it was probably the best that could be done under the circumstances, and yet it is inadequate. And it’s inadequate because it excludes all the exoplanets, limited to the solar system.

Deborah Byrd: Okay, let me stop you for a second before you keep going. Are you nervous about going back to the IAU now and presenting a new definition after all the uproar that happened in 2006?

Jean-Luc Margot: Well, I am not particularly nervous, but I sense that they (the IAU) are very nervous about it. I think there’s a lot of, the trauma is still very, very raw. And I think some of my colleagues in leadership positions are worried about this discussion, but you know, there’s a better way to do it, right? We don’t have to present the definition and say, hey, surprise, you know, here’s a proposal. So in fact, what we did months and months ago was to propose to the IAU, here’s a potential resolution for your consideration where we articulate the proposal. And the usual process for proposed resolutions is that they are posted on the IAU website at least six months prior to the meeting so that the community can read it, provide feedback, provide suggestions for improvement, and so on and so on.

Deborah Byrd: And so is that happening now? Are you getting that feedback now on your proposal?

Jean-Luc Margot: So again, for reasons that are not entirely clear, that did not happen. And instead, the IAU did not post our proposed resolution and asked us to come to the meeting and present it to have a large discussion and consensus before doing anything else. This is a little unfortunate because we know from 2006 that that’s not the way to do it, right? If there’s one way that you don’t want to define a planet is to bring a proposal without prior consultation and discussion with the community. So what we’re hoping to accomplish at this meeting at the most is to describe our proposal and possibly identify people who will be interested in improving it, refining it. And then at the next General Assembly in 2027 in Rome in Italy, perhaps the IAU will consider the resolution, post it on its website, and we may have a vote on it. We’ll see. It’s hard to predict what will happen.

Deborah Byrd: How many astronomers go to these meetings? How big is the organization that’s making this decision?

Jean-Luc Margot: So the organization was about 9,000 strong in 2006. I think at this time it’s about 12,000 strong. Don’t quote me on these numbers, but, and a fraction of the membership goes to the meetings. So I think 3,000 people probably attend the General Assembly. Now this is across all areas of astronomy, not just planetary science.

And one thing to note about the 2006 decision is that the bylaws at the time only allowed in-person voting. And so only the people who were present at the meeting and interested in voting voted. Now that has changed. The IAU has improved its process and now electronic voting is available. So I expect that if there’s a new resolution put forward, there will be a much wider proportion of members who vote.

Deborah Byrd: So what is the current definition of a planet?

Jean-Luc Margot: The current definition according to IAU resolution B5 in 2006 is that a planet must orbit the sun, our star in our solar system. That’s criteria number one. It must have sufficient mass to approach hydrostatic equilibrium, which means become rounded, roughly round, adopt a roughly round shape. Now it doesn’t say how round … This is currently very vague. Is the Earth round? Or is the moon round?

Deborah Byrd: Right, and some of the dwarf planets are oval-shaped,

Jean-Luc Margot: Some of them are round-ish. Again, it’s very vague. doesn’t specify. And the third criterion is that a planetary body must have cleared its neighborhood. Now, again, it doesn’t specify what that means, having cleared its neighborhood. Does that mean that there’s nothing left in the neighborhood? Does that mean that an occasional passing asteroid is okay or not? It doesn’t say.

And it also puts it in the past tense. You have an obligation to verify whether an object has actually cleared its neighborhood, which is very, very difficult to do. So it’s not only vague, but it’s also impractical.

Deborah Byrd: Right, because we’re in the process of being in an evolving solar system right now. So this whole process of clearing the orbit is still going on. I asked Mike Brown about that last week, because, of course, you could say that Earth has cleared its orbit, and yet objects still strike the Earth. So there are some, and he made the comment, and this is totally valid, that that’s part of the process of clearing the orbit, is that stuff, you know, smashes into other stuff.

Jean-Luc Margot: And if you imagine having to establish whether an exo-world has cleared its orbit, we do not have the technology to do that. It is completely impractical. And so we’re proposing instead to use the ability to clear an orbit over a specific time scale, which we propose could be 10 billion years or it could be the age of the universe. doesn’t really change anything. And so that’s a different concept having cleared an orbit, which is not observable, to an ability to clear an orbit, which is observable, because it depends only on the mass and the orbital period, and we can compute that. We can observe and compute.

Deborah Byrd: So in the press release that was released about this a couple of weeks ago, there was the word sun-centric, that the old planet definition is sun-centric. It’s my understanding that now you want to make the defintion less sun-centric, correct?

Jean-Luc Margot: That’s correct. We’re proposing that any planetary body that orbits either a star or a system of stars or stellar remnants or even brown dwarfs be acceptable as planets.

Deborah Byrd: Can we take those three one by one? So there are more multiple star systems, correct, in the galaxy, than there are single stars? More stars in multiple systems, correct?

Jean-Luc Margot: That’s correct. So we want to make sure that a planet that orbits a binary star can be counted as a planet.

Deborah Byrd: Right. And so brown dwarfs, those are objects that are sort of hybrid between stars and planets in mass.

Jean-Luc Margot: The canonical dividing line for brown dwarfs is about 13 Jupiter masses. And when an object has sufficient mass above 13 Jupiter masses, it can typically start deuterium fusion in its core. And those objects are then called brown dwarfs. Now it doesn’t have hydrogen fusion like the sun, and that’s what marks the beginning of a star.

And that requires something like 80 Jupiter masses.

Deborah Byrd: Right, so having hydrogen fusion is what enables stars to shine.

Jean-Luc Margot: That’s correct, yes.

Deborah Byrd: So, and then the third category of objects that was mentioned in the press release were stellar remnants. What do you mean by they could orbit, a planet could orbit a stellar remnant? What does that mean?

Jean-Luc Margot: Well, stars have very interesting lives and some of them end up as white dwarfs, other end up as neutron stars, other end up as black holes. We want to make sure that an object that’s in orbit around a neutron star could qualify as a planet.

Deborah Byrd: Okay, and what’s a neutron star?

Jean-Luc Margot: This is essentially a very, very high density end state of a star. This happens for the more massive stars, more massive than the sun. At the end of their life, when they run out of fuel, essentially undergo massive explosions and violent deaths and end up as a big pile of neutrons, essentially.

Deborah Byrd: Right. So the current definition is sun-centric, but the new definition would have that planets could orbit in multiple star systems. They could orbit these star-planet intermediate objects called brown dwarfs. Or they could orbit exotic objects like neutron stars. Could they orbit black holes? Could planets orbit black holes?

Jean-Luc Margot: Why not? Why not?

Deborah Byrd: Why not indeed? So that’s interesting. So that’s bringing the whole population of objects out there in the galaxy as potential home for planets, what we would hopefully now call planets. And part of your definition has to do with mass. So what’s the lower limit? Like how little mass can a planet have in your definition?

Jean-Luc Margot: Well, we have two proposals on the table. One proposal is aligned with all the IAU recommendations. So we’ve adopted the 2006 ideas, but we’ve made them more general, and we’ve quantified them. So we provide specific limits for what it takes to be round and for what it takes to clear a neighborhood. And in that definition, we provide a simple formula that you can use to compute whether an object has met this minimum mass criterion. In our solar system, it’s about 10 to the 23rd kilograms. But that could be different around lower mass stars or more massive stars, right? So that’s why we provide a specific formula.

Deborah Byrd: I see. And so how much is 10 to the 23rd kilograms? Can you give me a comparison of what that means? Is Ceres – the biggest asteroid – 10 to the 23rd kilograms?

Jean-Luc Margot: Ceres is about 10 to the 21st kilograms. So in our case, it would not be a planet, right?

Deborah Byrd: Okay, so it would not be a planet. It’s the biggest asteroid, so … yeah.

Jean-Luc Margot: Correct. Right. So one of the things we did in our paper was to try to motivate a classification system by doing a clustering analysis of solar system bodies. So we looked at their properties and then we asked essentially an AI tool to identify the various clusters. And when we do that, what we find is that satellites, planets, and other objects orbiting the sun planets form two distinct groups.

And then when we look at the set of objects that are either planets or [not planets], and we run additional clustering analysis based on mass or dynamical dominance [the ability of an object to clear its orbit], we also see gaps. And that 10 to the 23rd kilograms is right in the middle of a gap that separates the eight major planets from the other non-planets.

Deborah Byrd: That’s interesting. So before you did all of this, what you did was you looked at, you made a comparison of the known solar system objects and, with the help of an AI, you grouped them into groups. Is that what you did? Yes. And so you’re just trying to figure out, is there some natural way of collecting them into groups that would make these objects over here planets versus these objects over here that might be dwarf planets or asteroids?

Jean-Luc Margot: That’s right.

Deborah Byrd: I think it’s so interesting to talk about this nomenclature because it just seems like a big net of language. It’s like you guys are like these fishermen casting out this big net that’s just landing on these natural objects in the solar system. And they’re all so different from one another. But you’re trying to group them into these logical categories.

Jean-Luc Margot: Right. And what we found is that nature actually has created major gaps. So if we look at the clustering according to dynamical dominance – this ability to clear the orbit – we see a huge gap between the eight planets and all the other bodies in the solar system. It’s a gap of a factor of a thousand, right?

And this is really useful when you’re thinking about classifying bodies, right? There’s a gap that nature has provided for you right there, a dividing line. And it’s interesting also because nature doesn’t do gaps that are three orders of magnitude just willy-nilly.

Deborah Byrd: You wouldn’t think so.

Jean-Luc Margot: Something really fundamental must explain that gap. And so we’re trying to create a classification that matches that difference. And it allows us to think a little bit harder about the formation process of planets and other planetary bodies and what causes this difference.

Deborah Byrd: Okay, so one big difference between the old definition and the new definition is that the new definition is not sun-centric, and also in the new definition, an object has to be bigger than Ceres, which is now categorized as a dwarf planet, the asteroid Ceres, biggest asteroid, bigger than the biggest asteroid, but smaller than 13 times the mass of Jupiter. Yeah, 13 Jupiters is the biggest. And it can be and bigger than Ceres. So would Pluto go back to being a planet?

Jean-Luc Margot: No, Pluto is firmly in the cluster of objects that is not dynamically dominant. That is very clear from our analysis. Pluto, Eris, Ceres, all of these objects cluster in one group, and the eight planets cluster in a very different group.

Deborah Byrd: So Pluto does not clear its orbit, and that’s why it’s not a planet now. And according to the new definition, it won’t be a planet either.

Jean-Luc Margot: According to this proposal, a lot of discussion needs to take place and we’re hoping to encourage feedback from our colleagues about the proposal so that hopefully someday we’ll have a more precise definition and a more general definition of what a planet is.

Deborah Byrd: Poor old Pluto. Well.

Jean-Luc Margot: Well, Pluto is still tremendously exciting. Nothing has changed about how exciting Pluto is. The classification doesn’t tell you anything about the scientific importance of an object. It’s just a way of organizing our thoughts and of communicating. The fact that Pluto is not a planet doesn’t mean we shouldn’t go explore Pluto. We absolutely should.

Deborah Byrd: Absolutely. I mean, it has five moons.

Jean-Luc Margot: It has moons, it has amazing geology, it has this transient atmosphere, it’s a very dynamic place. It’s totally worthy of exploration. And the classification is, again, not an indicator of the scientific importance.

Deborah Byrd: Right. So are you expecting opposition? Do you think there might be people who would say, we don’t need a new planet definition or other kinds of opposition? Have you gotten any so far?

Jean-Luc Margot: You know, astronomers are human beings. They’re opinionated. I’m sure we will have all kinds of opinions. Some people may say that we don’t need a new definition. I think that’s a hard position to have considering how restricted our current definition is, right? We have thousands of exoplanets or exo-worlds. Are they planets or not? We don’t have a definition that applies to them.

So I think it would be hard to vote for the status quo, but some people might. I think scientists tend to be precise. So currently the definition is extremely vague. Again, some people might argue against being precise, but I think that’s a hard argument to make. Scientists tend to be precise about everything.

Physicists have definitions for force and energy and chemists have definitions for acid and pH. Why can’t astronomers and planetary scientists have a definition for a planet?

Deborah Byrd: Yes. And that whole idea of categorizing things into groups, that’s one of the first things that happens in science, isn’t it? When we’re exploring a new realm, we try to categorize things and see similarities between things and see differences between things. So it seems totally appropriate. And we have 5,000 exoplanets now. But when the Nancy Grace Roman Telescope goes up, I was told a couple of weeks ago on this live stream, we’re about to have 100,000 exoplanets.

Jean-Luc Margot: And that’s only the ones we know about. We expect statistically that there are hundreds of billions of planets in the galaxy alone.

Deborah Byrd: Right, just in our Milky Way galaxy. It’s mind-boggling. So you’re going to the International Astronomical Union meeting, the IAU General Assembly. And that’s around August 9.

Jean-Luc Margot: The meeting is August 6 to 15, and the presentation about our proposal is scheduled for August 9.

Deborah Byrd: I wish you the best of luck with it. It’s fascinating. Thank you so much for joining us. I’ve been speaking with astronomer Jean-Luc Margot of UCLA, and he will be proposing a new scientific definition of the word planet to the General Assembly of the International Astronomical Union in early August. Jean-Luc, thank you again. I hope we can have you back and find out what the progress on your planet definition proposal.

Jean-Luc Margot: Thank you so much.

Deborah Byrd: We’re EarthSky, and if you appreciate hearing from working scientists, please subscribe, like, and share. And don’t forget to join us live again next Monday, July 29th, when we’ll be talking about climate chaos with Dr. Daniel Swain. We hope to see you then.

Jean-Luc Margot and team published a new study in The Planetary Science Journal this past week (July 17, 2024) outlining their reasons for the suggested change in the definition to “planet.” Find their study here: Quantitative Criteria for Defining Planets.

Bottom line: When the International Astronomical Union created a definition for “planet” in 2006, Pluto lost full planet status. Now astronomers are proposing a new definition of “planet.”