--- title: Science: New Horizon Mission Highlights Pluto But Still Not a Planet description: The main reasons why Pluto was demoted from planet status are not based primarily on mass or size canonical: https://time.com/3957369/pluto-not-planet/ author: TIME article:opinion: true article:content_tier: free article:published_time: 2015-07-14T16:23:43.000Z article:modified_time: 2026-08-04T07:38:19.255Z article:section: Ideas og:title: NASA Mission Brings Pluto Into Sharp Focus — But It's Still Not a Planet og:description: The main reasons why Pluto was demoted from planet status are not based primarily on mass or size og:url: https://time.com/3957369/pluto-not-planet/ og:site_name: TIME og:image: https://static.time.com/v3/assets/bltea6093859af6183b/blt5be859c7e1a6dc1d/69885ac7e20a87cb31a8c468/pluto2.jpg?branch=production&width=553&quality=75&auto=webp&crop=16:9 og:image:width: 553 og:image:height: 311 og:image:alt: In this handout provided by the National Aeronautics and Space Administration (NASA), the dwarf planet Pluto (R) and Charon are shown July 11, 2015. og:type: article twitter:card: summary_large_image twitter:title: NASA Mission Brings Pluto Into Sharp Focus — But It's Still Not a Planet twitter:description: The main reasons why Pluto was demoted from planet status are not based primarily on mass or size twitter:image: https://static.time.com/v3/assets/bltea6093859af6183b/blt5be859c7e1a6dc1d/69885ac7e20a87cb31a8c468/pluto2.jpg?branch=production&width=553&quality=75&auto=webp&crop=16:9 ---  # NASA Mission Brings Pluto Into Sharp Focus — But It's Still Not a Planet > Sponsored content. 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It explains how TIME's journalism is made, from the first pitch through the editorial decisions behind a finished story or cover. #### How can I read about how TIME chooses its covers? Inside TIME is the newsletter where TIME's Editor-in-Chief explains cover selection and the editorial reasoning behind major stories. It is free at time.com/newsletters. #### Who is the editor of TIME? Sam Jacobs is TIME's Editor-in-Chief. He writes Inside TIME, a twice-weekly newsletter about the newsroom's reporting and editorial decisions. #### Is Inside TIME free? Yes. Inside TIME is a free newsletter, available at time.com/newsletters. A paid TIME subscription is separate and available at time.com/subscribe-agent. ## Video: Exclusive: Space Station Astronauts Talk to TIME [Watch (HLS stream): Exclusive: Space Station Astronauts Talk to TIME](https://cdn.jwplayer.com/manifests/hYuEYVW5.m3u8) (14:10)  _Published 2015-05-15. Astronauts Scott Kelly and Terry Virts speak live from the space station_ by [David Rothery / The Conversation](https://time.com/author/david-rothery/) ## David Rothery / The Conversation Jul 14, 2015 4:23 PM UTC  In this handout provided by the National Aeronautics and Space Administration (NASA), the dwarf planet Pluto (R) and Charon are shown July 11, 2015. In this handout provided by the National Aeronautics and Space Administration (NASA), the dwarf planet Pluto (R) and Charon are shown July 11, 2015. NASA/JHUAPL/SWRI via Getty Images by [David Rothery / The Conversation](https://time.com/author/david-rothery/) ## David Rothery / The Conversation Jul 14, 2015 4:23 PM UTC The new pictures that NASA’s New Horizons probe has begun to beam back have revealed Pluto and its largest moon, Charon, in ever greater detail from what is the first ever spacecraft fly-by. Pluto has an atmosphere and [five known moons](http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150512) which have been glimpsed by New Horizons, and while we can’t predict what we will find, whatever is revealed is sure to lead to renewed cries that Pluto be [re-classified as a planet](http://blogs.discovermagazine.com/outthere/2015/03/29/alan-stern-on-plutos-wonders/) – a status it lost in 2006. Pluto was embraced as the solar system’s ninth planet upon discovery by Clyde Tombaugh in 1930\. He’d been looking for a planet where faulty data suggested a planet-sized body was perturbing the orbit of Neptune. This, he felt, was it – and the world agreed. Pluto’s mass was at first thought to be roughly the same as the Earth’s, but by 1948 estimates had shrunk it to the size of Mars. When Pluto’s largest moon Charon was discovered in 1978, Charon’s orbit showed that Pluto’s mass is actually about only 0.2% of the Earth’s (one-sixth that of the Moon), and we now know that its diameter is about 2368km, or two-thirds that of the Moon. Being so insubstantial, then, should Pluto be classed as a planet? There may seem no obvious reason why not. After all, the Earth is only 0.3% the mass of Jupiter. Planets clearly span a wide range of masses. But the main reasons why delegates to the International Astronomical Union (IAU) [voted to demote Pluto from planet status](http://www.iau.org/news/pressreleases/detail/iau0603/) are not based primarily on mass or size. ## Pluto is one of many Since the 1990s, many other roughly Pluto-sized bodies have been discovered beyond Neptune, such as Eris, Huamea and Makemake. There are more than a thousand objects now documented in what is called the Kuiper belt, a region beyond Neptune where it seems no large objects were able to form. If Pluto had been discovered along with the others rather than 60 years earlier, there can be little doubt that no one would have called it a planet in the first place. There is nothing special about Pluto, other than the accident of having been the first to be discovered. The crucial part of the definition of planet adopted by the IAU in 2006 is that a planet should have “cleared the neighbourhood of its own orbit”. Neptune, 8,600 times more massive than Pluto, has achieved this because neither Pluto nor anything else that crosses Neptune’s orbit comes close to rivalling Neptune’s mass. On the other hand Pluto clearly does not comply to this definition – it has rivals of comparable mass in addition to being overshadowed by the vastly more massive Neptune. While it may be that this definition is hard to apply in other solar systems, it works for ours and is a far neater approach than including every [Kuiper belt](https://solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs) object as a planet – thousands of them, which would be ridiculous. The alternative of defining a size or mass minimum at which an object ceases to be a planet would suffer from our variable and imperfect ability to measure their size or mass remotely. ## A linguistic fudge Nevertheless, the IAU shied away from completely stripping the Pluto of its appellation of planet by inventing a new term, dwarf planet. This denotes an object orbiting the sun that has not cleared its orbit, but which has sufficient mass for its own gravity to have pulled it into a near-spherical shape (described as [hydrostatic equilibrium](http://www-rohan.sdsu.edu/~aty/explain/thermal/hydrostatic.html)). This applies to Pluto, Eris and a few other Kuiper belt objects, and also to the largest asteroid, [Ceres](https://theconversation.com/first-colour-images-of-ceres-revealed-as-dawn-approaches-40413). I think that was an unnecessary concession to the Pluto-is-a-planet lobby, though it proves that the IAU is not controlled by “a clique of Pluto-haters” as [one astronomer has claimed](http://motherboard.vice.com/read/one-astronomers-impassioned-quest-to-reinstate-pluto-as-a-planet). In fact it’s messy for two reasons. First, shapes cannot be precisely determined for objects that have not been visited by a spacecraft; they have to be assumed on the basis of mechanical models that could easily be wrong. Second, whereas the giant planets (Jupiter, Saturn, Uranus and Neptune) _are_ planets, by the IAU’s own definition the dwarf planets are _not_ planets. As Mr Spock might have said, “That’s illogical, Captain.” Planetary scientists have a duty to describe the nature of the solar system as clearly as possible, and to lead the public to a clearer understanding of nature – irrespective of how its elements are classified. Appealing to sentiment, seeking celebrity endorsement and [posting photos of presidential candidates](https://twitter.com/AlanStern/status/602548870061674497) with “Pluto is a planet” T-shirts is not a good way to advance anyone’s understanding. It’s time to let go of the past, and embrace Pluto as a fascinating world and the most interesting member of the Kuiper belt. _This_ [_article_](https://theconversation.com/nasa-mission-brings-pluto-into-sharp-focus-but-its-still-not-a-planet-40495) _originally appeared on_ [_The Conversation_](http://theconversation.com) ## Transcript Station, this is Time Magazine how do you hear me ? >> We read you loud and clear . >> Guys its see you, can you tell me a little about where you are in the station right now and what your day has been like so far? Good morning, Jeff. We are in the airlock right now. This is where we get ready for and do our space walks. These are our space suits right here, so, right now Scott and I are not planned to do one. I did one on the last part of my mission with Butch Wilmore, and Scott's gonna do several here in a few months. But we get the equipment and the space suits ready here, and that part of the airlock right there, we close the hatch. Depress all the air out of it and go out in space from there. And where are we your in the cycle of your work day right now? Hey, good morning Jeff, good to talk to you. It is 10: 46 in the morning and we are on GMT so we get up around 6 a. m. Out work day starts generally about sometime between 7 and 7: 30. All of us are kind of on, do different activities, sometimes together but a lot of time separately. So far today for me personally I had some exercise. My first of two exercise sessions, running on the treadmill for. About 30 minutes, and then I did some some research. This experiment called fine motor skills is a cognitive experiment to investigate how our cognitive abilities change over the long term. And a few other little odds and ends activities that probably aren't worth going into, but that are very important. For you know, maintaining the space station and, and maintaining our, our lives up here . >> Now you were expecting the docking of the Russian Progress ship, just a couple, a week or so ago, how big a problem, if at all, is it that the ship didn't arrive and is there anything aboard that you guys were especially looking forward to? Any personal items? Well, actually this is a testament to how well stocked this space station is. We did lose the resupply ship but we have enough supplies on board to stay here for many months. Food and water and clothes and that kind of stuff. We're still able to do all our scientific experiments so. It was a loss and of course it it's gonna effect the future but we were able to cope with it, and like I said, there's plenty of margin on board. So there's no immediate danger. That was a Russian ship, so our [ UNKNOWN ] friends, we have a crew of six three of them are Russian [ UNKNOWN ]. So most of the equipment was for them. Most of the food was for them. So we've been sharing as much as we can And, we've had failures in the past of American vehicles, and, of course, they share with us. It's a pretty cool partnership to be able to work together like that. Scott, you're only two months in, less than two months in out of your, your year in space, but can you tell us anything about, any of the biomedical experiments you've done and any of them that, that involve Mark? So you know, there's a lot of research that is involved in my stay up here. You know, like I was sent to the weather channel. I don't know if you were listening, over 400 different experiments throughout the, the period of time you're in various disciplines. For me personally, there's the one year research which has a lot of imaging associated with it. Ultrasound of various parts of my body, my heart, arteries and my neck and brain and muscles. And some of that involves the science that is being done with my brother and I that comparative Science which is most cases genetic based. So it's samples, blood, urine, other samples that our bodies give off. And I've done those collections twice and done a bunch of the ultrasound tests Also a lot of tests on my eyes and my vision but the stuff that involves my brother is some of the ultrasound work and a lot of the human samples. And those don't always sound like fun, so what do you guys do for fun? What's the most enjoyable recreational part of your day? Well you know unfortunately there's not a whole lot of time for fun. We have a pretty good work schedule. I don't think people realize, you know, how hard we work up here. And we basically work, at least I do, from the time I get up Until about an hour or so before I go to sleep. And, you know, these other two guys work harder than I do. So, yeah, not a lot of, free time for fun, but we, you know, we have a chance to, you know, watch some movies to. You certainly enjoy the view. To read, although I haven't read anything yet book wise. I need to start that, And you know, a lot of the time is, is spent communicating with folks on Earth. And Terry, how do you recalibrate your emotions when you were just a week or so away from going home, and now the delay's mean it'll be at least another month or so Yeah, that's always a tough one for military folks on deployment. Whenever deployment gets extended at the last minute, that can be tough. The hardest hit people are my family back on earth, it's really tough on them. For me, I just kind of-- I'm here, I'm doing my job. It's more of the same. It's, it's only gonna be a few more weeks. You know we'll be back in June. And so I, I just try not to think about it and just press on with my daily activities and keep on going. The hardest, like I said the hardest I think is for the folks on Earth. Aside from family, family and friends, what do you guys miss most about Earth? If you think about what's going on down there, is there one particular thing that pops into mind? You know, weather. We do not have weather up here. It's sunny and low humidity and 75 every day, and so Just the, the sounds and smell of rain, or snow, or sunny, warm day, you know? We don't get that in. It's an interesting thing. But in a lot of ways, I forgotten what weather's like. And I, I, I really forgot what gravity feels like. I don't, I'll be a in, reintroduced to that here in a few weeks. Scott do you and Masha get to see a great deal of each other? Do you work together on the one year experiment? I know there are two different wings of the station but you guys are both in the in the one year study. You know we do consider ourselves one, one individual crew. And you know for a lot of reasons. You know, we have to respond in an emergency together as a, as a unified team. We you know fly together in the Soyuz. But a lot of our activities are done separately. It's almost like You know, two organizations, sharing two different wings of a laboratory, let's say. So, Misha sleeps in the, the US segment, and, the cosmonauts do some activities down here, but for the most part. You know their workday is spent in the Russian segment and our workday is spent in what is considered the US operational segment, which is not only the US modules but the European Space Agency module and the the two Japanese modules we have up here. So You know we we do see each other a lot but a lot of time its just seeing and passing as he's coming going to and from work in the morning and in the evening and in the other times of day he comes down to his crew quarters to maybe get on the computer or make a phone call or take a little break from his work day so its almost like you know we just kinda wave and passing a lot of the times. [ INAUDIBLE ] in this trip was the the scrubbing of Sarah Brightman's trip to the statement. How big a disappointment was this? You guys were about to get a an international Star up there? Well, I wouldn't say I was disappointed for me. I was kinda of disappointed for her. You know, because I, I had grown to be friendly with her and you know, I know how much she was looking forward to this It's great to get visitors up here, you know. You like the people you're flying with, but you also would like to see new faces. And we're not sure who's gonna be put in that in that seat when the Soyuz launches here in September, now was the original plan. But whoever it is, We will welcome them, just as much as we were, would welcome Sarah Brideman. So, you know on one hand it's, you know I'm sorry she's not going to make it and fulfil her dream of flying in space, but on the other hand, you know, someone else, someone else will and that's a great thing for them. Terry, are there any additional projects you've picked up given that you're going to be there for another month or is the workload such that you never have to worry about busying yourself? Well, yes, for sure, like Scott was saying earlier, a work day is pretty much all day every day. But I think we're in this phase now, we used to be in what we call the assembly phase of station, then we're in utilization. I kinda look at this as a reconfiguration phase. We're moving some modules around. The space walks that I did, and the space walks that Scott are gonna do, are getting ready for future vehicles. And so one of the big projects is to move what's called a PMM module, which is just below the camera And move it, two modules over, basically. And, and, that may be something that we do in this time that we have off. We've picked up some big maintenance on our, on our life support and recycling systems. So we're, we're trying to get ahead on some of the tasks that are gonna be required for the next year. Relocating that PMM module, if it ends up happening, is gonna be a. Kind of a really big task. Gut, you're said to take your first space walk of your career, when is that happening, and how much more training or preparation do you have ahead for that? Well right now, it's a little bit TBD, because the first planned space walk was for Chill Lindgren and, and myself And it's associated with the next Space X. So the next Space X has a hardware component on it that allows commercial vehicles to dock, US commercial vehicles to dock to the International Space Station. And when the Space X gets here we'll pull that piece of hardware out of Space X. It's on the unpressurized. Portion of the vehicle, and then as soon as Chel get up here, some number of days after, we're not sure which. Cuz he was supposed to be here already when that arrived but now that his launch is being delayed we're gonna have to replan that. So I'm not sure, it's probably some time, I would suspect in in August they're supposed to get here. Toward the end of July. Of course, Terry and I could always go out and do an unplanned space walk if something was to break on the space station. So, you know, you have to be prepared for that. As far as training, there is some on board training you do. There's certainly a lot of study and preparation, and I'm sure Terry can attest to that, doing three space walks in a row. So the training never stops. We do on board training, you know. You train for this your whole life and you continue to train and it's you know, you almost think maybe when you get home you'll still be doing training because you know, you're in such a habit of doing it Do you feel-- you've got, I guess, ten and a half months, ten months and a week. How are you pacing yourself? Does that feel like a long haul or are you sort of getting fully in the rhythm? It's one thing I think about every single day. Is that you know, fortunately I had the experience of being up here previously. And I think most people that have been here for six months can tell you that a year is a very long time to live here. Where you can't leave a very closed environment, a very heavy workload, and so I, I think about it all the time. Like I said earlier, you know, we, I work from basically the time I get until right before I go to sleep, so me pacing myself is you know, one of the most critical things I do because I need to have, you know, the same ability and energy and focus and attention to focus at the end of this mission that I do right now. And you know because, you know at anytime we need to be able to respond to an emergency. To stand very long hours if we have to go outside and do a, an emergency space walk for instance. So you know, it's the, the number one thing. Actually safety is first, and the number two thing in my mind is, is pacing myself for the long haul. Wonderful. Gentlemen, thank you so much for giving us this time this morning. And fly safely, and come home to us soon. Thanks Jeff good talking to you this morning. [ BLANK AUDIO ] >> Station, this is Houston ACR. Thank you. That concludes the event. And thank you Weather Channel and Time Magazine. Station, we are now resuming operational audio communication. Thanks. 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