The Astronomy Thread

[icekatze]

hi hi

I'm personally rather partial to the classical definition of a planet, being a point of light that can be seen by the human eye, crossing the heavens against the background of stars at night. Mercury, Venus, Mars, Jupiter, and Saturn. But in the end, I suppose I am inclined to take more of a periodic table approach, and just expand the table when people start creating lots of new elements, like Plutonium.

Geomodder, the rest of the mass is not in planets, but in comets, satellites (moons, etc), meteoroids, and interplanetary medium.

GU Piscium b may be a brown dwarf if it is at the very high end of its mass estimate. Although, funny enough for this discussion, there is also some disagreement over the definition for where a brown dwarf begins and ends.

Regardless, the point is that the formation histories of various extrasolar systems are widely different. Our studies of other stellar proplyds and debris disks has revealed that they come in a wild variety of shapes and sizes. There are proplyd objects detected near Cygnus OB2 that are 50,000 to 100,000 AU across, which is apparently "in agreement with the theoretical scaling of proplyd size with distance from the ionizing source." HD 100546 has a binary disk, .2 to 4 AU, then after a gap, 13 to 100+ AU, with signs of a protoplanet forming at 47 AU. While Lynds 1551 IRS5's disk is apparently 20 AU across.

[Keter]

we're missing an admission that it's not 'Planet X', and that there never was one.

It's actually in the literature that all evidence so far is compellingly (conclusively) biased against a fringe solar system object at least as massive as the KBO mass ceiling ? My vague memory is that that's not the case.

[icekatze]

hi hi

Astronomers are understandably hesitant to make claims about possible fringe objects in the solar system after Pluto, but there are some models based on the orbits of Sedna and 2012 VP113, which suggest the possibility of one or more objects Earth sized or larger further than 100 AU from the Sun.

The Wide-Field Infrared Survey Explorer finished a survey of the local neighborhood in 2014 and didn't find any objects larger than Saturn at 10,000 AU or closer, an no objects larger than Jupiter at 26,000 AU or closer. So there is an upper limit on the possibilities, but WISE isn't able to detect particularly cold objects, so there is still areas left unexplored. (An Earth sized object beyond the Kuiper belt would be an estimated 35 kelvin, below WISE's 70-100 kelvin limit. And it would be classified as a dwarf planet.)

[Arioch]

The DSCOVR satellite, positioned at the Earth-Sun Lagrange 1 point, records a lunar transit across the Earth's disc, revealing the fully-lit far side of the Moon.



http://www.nasa.gov/feature/goddard/fro ... e-of-earth

[icekatze]

hi hi

Nice catch! The moon really is quite dark compared to the earth, It's hard to think of it that way when it is so bright in the night sky. It is a shame about the faint RGB artifacts in the image, but I guess it is still pretty impressive, given that each channel is taken 30 seconds apart.

[Arioch]

The moon really is quite dark compared to the earth, It's hard to think of it that way when it is so bright in the night sky.

That struck me as well, and one of the researchers in the article mentioned it also. Earth is very bright.

[Razor One]

Yeah, it only reflects 1% of the light that it receives. I recall reading somewhere that if the moon were as bright as Earth was square meter for square meter, it'd be bright enough that you could read by its light.

It only appears as bright as it does to us because it tends to be the biggest and brightest thing in the sky, and your pupils are usually dilated more when you're out at night than in the day, making it seem brighter than it really is. Consider just how faint the crescent moon looks when it has to compete with the sun when they're both up in the sky during the day.

[dragoongfa]

I think that I read somewhere that our kind of atmosphere is very reflective and this is one of the reasons as to why life flourished here at the first place.

[Krulle]

Yet, the "dark side" of the moon is the brighter of the two sides....

[GeoModder]

Yet, the "dark side" of the moon is the brighter of the two sides....

Yes, less mare on the far side.

[Arioch]

Yeah, it only reflects 1% of the light that it receives.

More like 10%. The Moon's albedo is usually given as about 0.11, and Earth's is around 0.39 (though these numbers include non-visible wavelengths).

[Razor One]

Yeah, it only reflects 1% of the light that it receives.

More like 10%. The Moon's albedo is usually given as about 0.11, and Earth's is around 0.39 (though these numbers include non-visible wavelengths).

I really should've researched that before opening my big fat gob. That or the source I read was talking about visible wavelengths only.

Ah well. I don't mind being wrong about space since that means I learn new things.

[icekatze]

hi hi

An interesting find, it seems that the gemini planet imager has gotten a direct image of a planet orbiting Eridani 51 that is likely only 2 times the mass of Jupiter.

Oh, it is also only 20 million years old. Might just provide some insights on how our own solar system formed.

[icekatze]

hi hi

Another interesting find. The recently discovered Kepler-453b is a planet in a stable orbit around two stars in the habitable zone.

[Arioch]

This is a cute visualization of the scales of the various Kepler planetary systems compared with our solar system. I think what's interesting is how small most of them are.



This is of course an artificial selection effect resulting from the detection method. Transit detection can really only find planets that are very close to the primary, both because at greater distances even a minor orbital inclination will take the planet out of the visual plane, and because Kepler's limited (~5 year) observation time was not sufficient to detect any planet with an orbital period of more than a few years. So the largest orbit you see here is only a little bit farther away than Mars. Still, it's remarkable to see how many planets (and how large planets) can be crammed into such very close orbits.

[Mjolnir]

It's only usefull for the general education of the public, astronomers make very little distinction between big bodies and small bodies of the Solar System.

I think this is the larger point: the definition has almost nothing to do with science. And it's confusing to the public, because they mistakenly assume that it does. Telling the public that Pluto is no longer a planet does more harm than good as far as general education is concerned.

The chosen terminology doesn't help things. A category named "dwarf planets" that is not a subset of the category "planets"? Dwarf stars are stars, giant stars are stars, giant planets are planets, dwarf planets are...something that we call a planet while insisting it's not a planet because of factors that don't actually have anything to do with the object we're classifying? Who thought this was a good idea?

[icekatze]

hi hi

Maybe when we invent time travel, we can go back and determine which planets cleared their orbits, and which ones got help from Jupiter.

Also, check out these sweet sweet high resolution photos of Pluto's surface geology. Craters with stratified layers indicate that it is not just one big homogeneous clump of debris.

[Absalom]

It's only usefull for the general education of the public, astronomers make very little distinction between big bodies and small bodies of the Solar System.

I think this is the larger point: the definition has almost nothing to do with science. And it's confusing to the public, because they mistakenly assume that it does. Telling the public that Pluto is no longer a planet does more harm than good as far as general education is concerned.

The chosen terminology doesn't help things. A category named "dwarf planets" that is not a subset of the category "planets"? Dwarf stars are stars, giant stars are stars, giant planets are planets, dwarf planets are...something that we call a planet while insisting it's not a planet because of factors that don't actually have anything to do with the object we're classifying? Who thought this was a good idea?

Someone who metaphorically wants magical unicorns to exist in space, I assume. It's fairly easy to come up with a sensible description of the low-end of a planetary mass based on the self-attractive force required to pull it into a spheroid despite some particular starting conditions (on average 0 C or something, whatever), but apparently they wanted planets to be rarer and more "special" than that: all that's "special" with what they produced is the justification for it's production.

[joestej]

Someone who metaphorically wants magical unicorns to exist in space, I assume. It's fairly easy to come up with a sensible description of the low-end of a planetary mass based on the self-attractive force required to pull it into a spheroid despite some particular starting conditions (on average 0 C or something, whatever), but apparently they wanted planets to be rarer and more "special" than that: all that's "special" with what they produced is the justification for it's production.

I believe the justification for reducing Pluto from planet status is because if they allowed it to remain a planet, we would have to add Eris, Quaoar, and Sedna to the planetary list as well. And Eris is actually BIGGER than Pluto.

There are currently three qualities required to be a planet:

It must be in orbit around the Sun
It must have sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
It must have "cleared the neighborhood" around its orbit (meaning there is nothing in its orbital zone similar in size beyond the object's own satellites)

Pluto meets the first two but its orbit is littered with objects relatively close to its size. There are so many they even have a special name: plutinos.

I suspect the reason 'dwarf planets' are called dwarf planets instead of something more accurate is to pacify the legions of ordinary people who were up in arms that Pluto would no longer be a planet. But we must face facts that science has marched on. If the people who study astronomy for a living think that something really shouldn't be called a planet I'm certainly not qualified to tell them they are wrong.

[icekatze]

hi hi

When scientists started discovering and creating new elements, like Nobelium, they didn't say "Oh my gosh, there are too many elements on the periodic table, we need to reclassify some of these, and besides they decay too fast to be important anyway." They just expanded the periodic table.

The "cleared the neighborhood," component, as has been discussed at length already in this thread, is a terrible measurement. We can't even agree on models of how the solar system formed, let alone which planets did what. (According to the nice model, which has already been discussed, interactions between Jupiter and Saturn were responsible for clearing the orbits of the entire solar system, including Neptune and the inner planets.) After all, the further out you get, the area that is required for a planet to clear grows exponentially, while the effects of gravity trail off exponentially. So it is quite likely that even Neptune would not be able to clear its own orbit by itself.

The people who study astronomy for a living have not reached a consensus on the matter, and it continues to be a point of contention, in spite of what a small handful of people voted on. Not to mention being entirely inapplicable to solar systems beyond our own.

Science is not decided by vote.