aesmael

Originally published at a denizen's entertainment. You can comment here or there.

First of all, nifty article from Bad Astronomy on the Moon's youngest known natural crater. This was not present in Apollo 15 images from 1971, but was present in images from the Lunar Reconnaisance Orbiter dating to 2009. So that is very new! The actual object responsible would have been tiny, and harmless to Earth thanks to our atmosphere, but still. Isn't it cool that things still change in the solar system? Not even our moon is static.

Second of all, I don't know anything about them but these planned ESA missions sound like they could provide us with a lot of exciting information once they get underway. Particularly the LISA precursor - I'd had the impression that LISA, which might get us our first direct detection of the gravity waves predicted by general relativity, had been canned.

Finally, here is a pretty video of the sun:

Originally published at a denizen's entertainment. You can comment here or there.

A fun paper I came across a while ago - The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space. The question at hand is, what are the optimal conditions for a planet ejected from its star system to retain liquid water?

Unfortunately it has been a few months since I read through the paper itself, so I've forgotten a lot, and I'm not willing to do it again just now. However, the results are surprisingly optimistic. A terrestrial planet with an Earthlike composition would be able to retain liquid water under a layer of insulating ice for significant periods if its mass were approximately 3.5 times that of Earth. And since more massive planets could potentially retain a higher mass of volatiles I wouldn't be surprised if isolated planets below that threshold retained liquid water too.

Meanwhile, this sentence - "If a rogue planet had about ten times higher water mass fraction or a thick cryo-atmospheric layer, it would need to be only ~0.3 times the mass of Earth to maintain a liquid ocean." - seems a bit more theoretical, since I am not sure are likely to encounter planets of such low mass with such large amounts of the appropriate materials. Unless I un-forget about the possibility of predominantly icy planets.

The whole paper is fascinating and worth a read. It's just a shame it is so difficult to detect these planets if they are out there. I am not sure we ever will without some form of interstellar travel (if nothing else, even slow automated exploration would give our descendants more vantage points from which to potentially observe one). Or we could get lucky and find one in the neighbourhood. Not betting on that.

Originally published at a denizen's entertainment. You can comment here or there.

I disagree with the IAU's decision on defining planets, sure. But articles like Should The Cluttered Skies Demote Earth? (and comments of a similar style) make me want to ask people to please stop helping.

Pluto and other 'dwarf planets' have crossed a threshold. Their mass is sufficient that gravity[1] dominates their shape. They are probably all differentiated bodies. So I consider them interesting in ways different to other small bodies, the smaller asteroids, comets, kuiper belt objects, and irregular moons. Besides, in astronomical terms our sun is a dwarf star, but it is still a star.

However. If one looks at the dynamical properties of these bodies, differences between these 'dwarf planets' and the others become sharply apparent. They just don't have the same heft. Looking at the table in that article it is clear that one of the most common arguments against the dwarf planet categorisation - that if you put Earth or some other planet out in the kuiper belt, it would be called a dwarf planet (and that location should not determine what an object is) and thus the whole dwarf planet thing must be nonsense. But according to those figures Earth would have to be moved nearly 3,000 times further from the sun before it would even approach the dynamical insignificance of Pluto or Eris, and that doesn't account for the effect Earth would have had on that region had it been orbiting out there these several billion years - I am pretty sure those figures are derived from the solar system as it is now, not as it might have been.

I do think for planetary bodies a combination of physical and dynamical properties ought to be considered when deciding what to call them. If Ganymede or Titan orbited the sun we'd probably call them planets. If Mercury orbited one of the other planets, we'd probably call it a moon (actually we might call it a double planet if it orbited Earth or anything smaller, but never mind). So I think location matters. Context matters. And arguments like "If we moved X it would / wouldn't be a planet" aren't convincing because they're trivial or false. Nor, to say Earth should be a dwarf planet on account of the stuff around our orbit because the total mass of it is insignificant - in any interaction between them Earth will do the scattering, not be knocked around or much affected. As arguments go, they're pathetic, and I'm embarrassed to see people somewhat on my side using them.

There is an article by S. Alan Stern and Harold F. Levison proposing a planetary classification scheme which meets all of my requirements, and adds some new dimensions I had been hoping for but had not thought of. Although I am partial to major and minor planets rather than über and unter planets the latter set is probably a better choice for not conflicting with our existing use of minor planets. I recommend reading the linked pdf for anyone interested in this topic. It is clear and straightforward, and I doubt I could express it better here. If I were Tyrant of Astronomy I might make that document law and then abdicate. Maybe.

I keep trying to write a description of the proposed scheme here despite what I said immediately above. Probably because I worry people won't follow the link. So, short of copy-pasting large chunks, we simplify: a planet is defined as an object massive enough that its shape is dominated by gravity, but which does not ever sustain fusion. A mathematical test is proposed to decide whether a particular planet is über (dynamically dominant) or unter (not dynamically dominant) in its orbit. Some planetary satellites are considered planetary bodies themselves. Further classification subdivisions are proposed according to composition (rock, ice, or hydrogen) and mass (subdwarf, dwarf, subgiant, giant and supergiant[2]). In this scheme, Earth would for example be a rocky dwarf über planet.

Much of what pleases me about this scheme is its scope for recognising both the variety and the similarities of planets. Pluto is different from Mercury is different from Earth is different from Uranus is different from Jupiter. They're all different from Titan, Triton, Mars and Ceres. But they're all interesting, and have qualities in common which non-planets do not have. Plus, it feeds my sense of wonder and excitement about the universe.

[1] Along with other details such as their material properties and angular momentum, e.g. 20000 Varuna. Also happens with stars, for example Achernar.

[2] This is fairly directly taken from how we classify stars according to size.

Originally published at a denizen's entertainment. You can comment here or there.

Increasingly less recently I had been discussing with Pazi a game project. I wanted to do something useful so I offered to craft a little program module which could generate 'realistic enough' star systems. Something that would not simulate stellar formation and evolution directly, but which could use information derived from those simulations. I suspect such a tool could find a few uses, all of roughly the same sort, but apart from being useful I mainly think it a pretty nifty idea. Of course I would need to first finish my current project, but that is badly in need of finishing anyway.

Trying to determine what I would consider a 'first finished' version, not the first steps I would take in writing it but what it would need to do for me to consider it getting to completed. That, and some sorting out in my mind of how to approach the workings of this thing.

My thinking is to start with the star itself and build up the system around it in a sort of abbreviated history. Presumably, get it working for single stars before worrying about multiple systems, unless those turn out not really harder to put in. We get a few basic figures for the star, stuff like mass, spectral type, luminosity, then use those to proceed. Stellar mass typically correlates with disk mass available immediately after its formation, determining how much material is available to form planets and potential early migration activity for them. Plus, the distances at which various sorts of planets can form. I think we can relate those figures to get some nice functions for populating planets from. Start by placing giant planets, unless consensus has changed on the order of formation, then the little ones, abstract some pseudo-orbital-evolution and there we have our own impromptu planetary system.

Each planet's numbers - semi-major axis, eccentricity, mass, composition - get to interact with the host star's numbers, and we get a rough guide to the world's likely surface conditions, atmosphere and chemistry of note. There might be more than one class a given world could fall into.

I find the prospect of this fairly daunting. I imagine for an experienced programmer it would fall into the class of 'relatively simple' but for me, it's not. I'm only just barely learning, still. But I look forward to accomplishing this. I suspect the first thing needing doing is collecting papers with relevant modelling statistics to use; been wondering if it might be much harder too to get information for moons and small bodies.

I worry I have somehow done badly by describing my goals in the terms I have, rather than some technical description of coding intentions, but suspect this to be a silly worry. Am expecting to be learning a lot of how in the doing, and believe it is advantageous to have clear ideas on what I want the outcome to be.

Once that 'first finished' version is working I would want to add some fancier abilities, like aging. This would be needed anyway to incorporate giant stars since those again modify their planets as they age and expand and brighten, but also would be useful for incorporating very young stars still in the process of forming planets and details like at what point terrestrial worlds are likely to become tectonically dead. I suppose an approach that might work is to generate values for the whole system's duration (at least, mark points when things change and the values before and after). I suppose it is unlikely this program would get used for anything that depicts time-spans long enough for stars to visibly age, but at least for some sort of manual editing or inspection it would not end up generating a new history every time the age is tweaked. Perhaps too much, but I cannot help but feel it would be a useful approach somehow.

As for populating a larger 3- or 2-dimensional space, I suspect that would be harder to do, maybe want for another tool that talks to this one. Currently I'm thinking finding a good way to place stars in a field would be a more difficult thing to do than getting population kind distributions for various stellar environments. Hopefully by the eventual time I get to attempting that I will know much better.

That's main thoughts and plans on this for now.

[At Systemic can be found an excellent example of why this will likely be difficult to do. At the moment our models aren't reliably predicting the sizes of giant planets in some important situations. And we're still finding situations where our models are in error, which is why I suspect I will have to aim for 'realistic enough' and post refinements as astronomers make refinements. Or as soon after as I am able to.]

Originally published at a denizen's entertainment. You can comment here or there.

In the past week I have been surprised by two pieces of news concerning Saturn and rings.

First, from the Planetary Society Blog: findings which might be evidence for a posited ring around Rhea. As described in that article, a series of equatorial spots on Rhea bright in ultaviolet light might be evidence of collisions from ring particles orbiting the moon. These particles, if they exist, would occupy a size and distribution which makes them particularly difficult to detect visually directly with the instruments we currently have available.

It seems the idea of rings around Rhea has been around longer than I have been aware of. Apparently they were originally proposed to explain a decreased electron flux in the vicinity of Rhea back in 2005, and I sure didn't realise there was this much evidence already. Would be very exciting indeed to get a direct and definite confirmation about this.

Sadly given how difficult these rings are proving to image, it is unlikely there will ever be beautiful views of the Rhean ringscape. We shall just have to comfort ourselves with the knowledge of something wonderful.

The other news is the discovery of a new ring around Saturn itself. This one, discovered by the Spitzer Space Telescope, is the largest and most diffuse planetary ring yet discovered. The details can be found in this press release. Basically it is very large and very faint, and only detected because of its cool infrared glow. I am concerned that the end of the release specifies this information was gathered before Spitzer ran out of coolant, and whether this means we won't be able to obtain further observations of the ring for a while. It might take us a long time to discover if this ring 'only' spans from six to twelve million kilometres from Saturn, or if that were merely its brightest, densest part.

Apart from the amazement of a whole new feature being discovered, this is particularly intriguing because Saturn's moon Phoebe orbits within this ring and is thought to be the source of its material (via dust knocked off from impacts, most likely). If so, and depending on what else is found, this ring could be a key part of one of astronomy's longer-standing mysteries: the two faces of Iapetus. Although it has long been suspected that material from Phoebe deposited on Iapetus is the reason that moon has one bright hemisphere and one dark (more or less), I think this ring is the first actual detection of a possible mechanism for the transfer of this material.

If that bears out I think I will finally have an answer to a mystery I have been intrigued by since I was a young child; for a long time Iapetus has been one of the solar system bodies I was most fascinated by.