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Archive for the ‘exoplanets’ Category

how to define a planet: the problematic case of Pluto

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Pluto, with its ‘heart-shaped’ area known as Sputnik Planitia, imaged by New Horizons, July 14 2015

A while back I listened to a podcast from Point of Inquiry, in which two planetary scientists, Alan Stern and David Grinspoon, involved in NASA’s New Horizons mission to Pluto, were separately interviewed, and were inevitably asked about Pluto’s demotion from planet status. Having not followed this issue, I was surprised at the response. So it’s time to take a closer look.

Of course I should be writing ecstatically about the New Horizons mission, not to mention those of Juno, Cassini, Mars’ Curiosity and so forth, and hopefully that will come, but the controversy about Pluto immediately struck me, as I thought, in my naïveté, that its demotion was a consensual thing amongst astronomers, with only the ignoroscenti (my neologism) left to mourn the fact (not that I mourned it particularly – Pluto still existed after all, and it didn’t care a jot what we thought of it).

Pluto, discovered by Clyde Tombaugh in 1930, was accepted as the ninth and final planet in our solar system for decades until the nineties, when another Kuiper belt object was discovered (besides Charon, Pluto’s large moon), and the Kuiper belt itself became a thing, in fact a massive thing, far bigger than the ‘familiar’ asteroid belt between Mars and Jupiter. We now know of more than 1000 kuiper belt objects, with at least 100,000 believed to exist. The Kuiper belt is widely spread out from the orbit of Neptune, and though Pluto is its largest and brightest object, it’s not the most massive. Presumably it’s for this reason that Pluto was demoted – what with the scattered disc and the Oort cloud there seemed to suddenly be a host of objects that could be included as planets, so it was thought better to exclude Pluto, or to demote it to dwarf planet status, presumably along with other assorted Kuiper belt objects (KBOs), rocks and iceballs that were worthy of the designation. That seemed okay to my thoughtless mind, but here’s what Alan Stern had to say on the subject:

Well, you know, we don’t really honour that classification in planetary science, that was really done by a group of different astronomers who don’t know much about planets. Let me give you a technical term, we call it BS. You know what BS stands for don’t you? Bad Science. Now you wouldn’t ask a podiatrist, a foot doctor, to help you if you had a cardiovascular problem with your heart, that’d be the wrong expertise, though they’re both doctors you’d be going for a cardiologist. And if you had a real estate problem you probably wouldn’t go to a divorce attorney, even though they’re both attorneys. In the space field we have many professions, we have engineering professions, we have many different scientific specialties, etc. Astronomers really don’t know much about planets any more than I’m an expert in black holes in faraway galaxies. They had a little meeting in 2006, they were worried that school children would have to memorise the names of too many planets, so they wrote a definition that limited the number of planets to eight. Now, right after that, Ira Flatow called me up on Science Friday and said, would you debate Mike Brown, who was one of the proponents of ‘let’s limit the planets to eight’, and I said, sure, and we got on the phone and it’s Science Friday live, and Mike Brown makes his case and says, ‘look we just can’t have 50 planets, it’s too many to remember.’ Now, I found that anti-scientific, it seems like engineering the definition, versus letting it inform you, but Ira said, Alan what’d you think, ‘can’t have 50 planets’, what d’you say back to MIke? I said, ‘well if you can’t have 50 planets then we’re probably going to have to go back to eight states, I guess’. And he was speechless…

I love that story – though no doubt Mike Brown would’ve told a different one. So let’s turn Stern’s objection into an inquiry. Was it scientifically correct/accurate/fair to reclassify Pluto as a dwarf/minor planet?

Happily I just happened to listen to a podcast of the Skeptics’ Guide a few days later, which has led me to a more detailed piece on Steven Novella’s Neurologica blog on the Pluto controversy. Apparently, in the above-mentioned 2006 meeting they decided that to be classified as a planet, a body in our solar system should meet 3 criteria:

  • it has to orbit the sun
  • it has to be spheroid (i.e. have the mass to be so, due to its gravity),
  • it must have cleared its orbit of other objects.

Now this third criteria immediately seems the dodgiest, as it sounds like it’s designed to eliminate any KBOs. And how do we know an orbit is cleared? After all, one day, a comet or asteroid may strike us, because our orbits have coincided this time around. And why is that third criterion even important?

Novella cites a recent paper by planetary scientist Phillip Metzger who argues that the third criterion is invalid and that nothing about a body’s orbit should be in the definition since orbits can alter due to external influences. Only characteristics intrinsic to the body should be included in the definition. This would essentially leave one criterion standing – that of sphericity. And even then, how sphere-like does a planet have to be? Another ‘problem’ with Metzger’s definition is that it would include moons, such as our own, and many others. Novella has his own classifying suggestion, which sounds promising to me:

We keep criteria “a” and “b” and drop “c”. However, we add that the object must not be in a subservient orbit around a larger object. What does that mean? If two objects, like the Earth and Moon, are in orbit around each other, and the center of gravity (barycenter) lies beneath the surface of one of the bodies, then the smaller object will be said to orbit the larger object, and is a moon. Therefore Europa, which is large enough by itself to be a planet, would instead be considered a moon because it orbits Jupiter.

I need to further explain the term ‘barycentre’, for my own sake. Think of two bodies in gravitational relationship to each other. Inevitably, one of them will be more massive, and will exert a greater gravitational force. An obvious case is the Earth and the Moon. Between the two there is a point, the ‘centre of gravity’, or barycentre,  around which the two bodies revolve, but because the Earth is a lot more massive that the Moon and they’re relatively close to each other, that barycentre is actually close enough to the Earth’s centre to be within the mass of the Earth, with the result that only the moon revolves. The Earth, though, is very much affected by the Moon’s gravitational field, which causes a slight wobble as well as tidal effects on the Earth’s surface. 

Interestingly, Novella’s reclassification would include Charon, Pluto’s ‘moon’, as a planet (as well as Pluto of course) because its size relative to Pluto puts the barycentre at a point between the two bodies, rather than within Pluto. So Pluto-Charon would be reclassified as a binary-planet system. It would also promote Ceres, in the asteroid belt, and Eris and Makemake, two recently discovered Kuiper belt objects, to planetary status. That takes the current eight up to thirteen, with others yet to be discovered. 

It’s unlikely of course that the astronomical overlords who reclassified Pluto would be swayed by any mere outsider’s view, however well-reasoned, but this examination of the issue is a reminder of just how dubious the reasoning of ‘experts’ can be, and how important it is to question that reasoning. Size apparently does matter to these guys, but this new category of ‘dwarf’ or ‘minor’ planet seems inherently unstable, and will probably become even more so as the number of discovered exoplanets increases. Will it be mass or volume that’s the decider, and what will be the mass or volume that decides? And does it really matter? It’s only nomenclature after all. And yet… The difference between an asteroid and a comet is important, is it not? And so is the difference between a planet and an asteroid. And so is the difference between a moon and a planet. And so… is it not? 

Written by stewart henderson

October 14, 2018 at 1:09 pm

Proxima b

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Quote of the day/week/month/post:

Better to have questions you can’t answer than answers you can’t question – Max Tegmark (and many others)

proxima_system

Jacinta: So while astrophysicists argue over the likelihood of life elsewhere in our tiny but massive universe, some are focusing on our nearest star neighbour. Some wobbling of the red dwarf known as Proxima Centauri has revealed, upon lengthy observation, that it has a closely orbiting planet, which considering the relative coolness of the star – way too dim to be seen with the naked eye – and the proximity of its satellite, is very much in the habitable zone. While it’s too early to say so much for the naysayers, the discovery of a planet in the Goldilocks zone of our nearest star in a galaxy of billions of possibilities must surely raise hopes and expectations of life abundant.

Canto: This closest possible exoplanet was only discovered in August this year, so we’re desperate to find out more about it. Being in the habzone is one thing, habitability is another. Obvious questions we have no current way of answering are: does it have an atmosphere? Any possibility of water? Is it tidally locked? And of course we’d love to know if we could launch some sort of robotic mission to our nearest star neighbour. Meanwhile is there any other way of gleaning more info from this tantalising object?

Jacinta: It’s not likely to be habitable though. Solar winds are estimated to be some 2000 times those experienced on Earth, though we can’t be too sure. Researchers are trying to work out the size of the planet…

Canto: How do they know about those solar winds?

Jacinta: Oooh, that’s a horribly good question. It’s due to the closeness of the orbit, where you would expect the solar winds to be much stronger, as they are in our solar system. It’s believed that Mercury’s magnetic field, which should be stronger than it’s been measured to be because of its heavy metallic core, is dampened massively by our solar wind. So basically they would’ve inferred Proxima Centauri’s wind by our own. As to how they came up with the figure of 2000 times that experienced on Earth, I’ve no idea, but strong solar winds make it hard to maintain an atmosphere, which is vital for life. You’ve also talked about tidal locking, which is a feature of close orbits, such as the Moon’s orbit of the Earth. So you’ll have a permanently hot day side and a permanently cool night side, and this can be problematic for the creation of an atmosphere, according to modelling.

Canto: Now, all of this sounds very negative, but basing exo-planetary activity on what’s been the case, as far as we can work it out, in our solar system, has been really problematic hasn’t it?

Jacinta: Definitely, that’s why we need to go beyond modelling, if we can, and collect some real data. So we’re looking to the James Webb Space Telescope (JWST), the very exciting successor to Hubble to be launched around November 2018, to garner more info, which it’ll be perfectly equipped to do.

Canto: If by some near-miraculous combination of circs there is an atmosphere on Proxima b, or a reasonable quantity of liquid water, that would help distribute the heat around the planet. With no atmosphere, the difference between day side and night side would be stark.

Jacinta: Exactly, and that’s what the JWST should be able to detect, as the best way to detect the atmosphere is to measure the planet’s infrared heat signature. If the JWST finds a decisive and fixed difference between the planet’s day and night sides, it’s a safe bet that no atmosphere is present. The JWST will be equipped to measure this IR signature on both sides of the planet, and if it doesn’t find that stark difference, that’ll be when we can start speculating about an atmosphere and its constituents.

Canto: Though of course they’ve already started with the speculation. But really, whatever they find – and I don’t expect that everything will line up for life – the fact that we’ve found an exoplanet well worth investigating on the nearest star outside our solar system, with billions of stars yet to be homed in on, one by one – doesn’t that say something to those who argue for the Fermi paradox – where are they? Okay, Fermi and Hart were talking about intelligent life, and that may well be orders of magnitude more difficult to develop than life itself, but I’m sure that Fermi would be unsettled in his skepticism, if he was alive today, by the vast numbers of exoplanets, in other words possibilities for life, we’re discovering now, with so many to come in the near future.

Jacinta: Yes, bliss in this time it is to be alive, but to be young, that would be very heaven!

 

 

References:

Cosmos issue 71, pp9-10

http://www.gizmodo.com.au/2016/08/how-well-get-our-first-big-clue-about-life-on-proxima-b/

en.wikipedia.org/wiki/Proxima_Centauri_b

 

 

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Written by stewart henderson

December 4, 2016 at 9:38 pm