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'Homeless' Planets May Be Common In Our Galaxy
sciencehabit writes "Our galaxy could be teeming with 'homeless' planets, wandering the cosmos far from the solar systems of their birth, astronomers have found. In a paper published online today in Nature, the researchers list 10 objects in our galaxy that are very likely to be free-floating planets. What's more, they claim that in our galaxy, free-floaters are probably so populous that they outnumber stars."
Which administration gets the blame for that?
--- Asking inconvenient questions for over 30 years...
I guess the economy's bad *everywhere*.
Sheesh, evil *and* a jerk. -- Jade
tomorrow we will have homeless moons, rocks, asteroids etc etc etc... and dark matter will be reduced in a big 0.000001%.
Except that they have to account for five times as much as what astronomers can see or infer exists.
Did it ever occur to you that the experts might actually know what they're talking about?
Sheesh, evil *and* a jerk. -- Jade
Well, while I wouldn't think they'd be "space ships" in the classical sense.
I do wonder, would this not be a viable method of extremely long term, interstellar travel?
Find a "free-floater" (terrible name), build a perhaps subterranean civilization, somewhat colonize this planet, impart an impulse, and go for a ride for millions of years. Given we're advanced enough to even make it to one, we might even be able to attach "weak" but sustainable engines to it, such that we can slightly control it. It wouldn't be a terraformed planet, or similar, more like a moon, which we can live on, sustainably, regardless of the vacuum of space, and lack of sun. This would then essentially be a giant, "space ship".
Interesting idea.
This is my footer. There are many like it, but this one is mine.
A bunch of planets floating around in space without orbiting a star
Come to think of it, why are we working on the assumption that basically every object in the universe is on fire*? As a personal bet, £5 says that the vast majority of mass in the universe is in solar systems where the central object either wasn't on fire, or has burnt out billions of years ago
* Yeah, AFAIK the official explanation for this assumption is "because that's all we can see", but maths says there must be more - the explanation for the rest always seems to have dark matter being some mysterious meta-mass unlike anything we know about, rather than simply being planets with no nearby light source...
And your PHB says it shouldn't take so long to write programs.
How come everyone around here knows more than the experts? Do you really think astronomers are too dumb to think of all the non-stellar matter in and between the galaxies?
Sheesh, evil *and* a jerk. -- Jade
Tch, they're not really planets, right? I mean, if they're not orbiting a star, then they can't have "cleared the neighborhood of their orbit". Yet one more reason the IAU's current definition is so idiotic. (Besides the fact that it suggests that Mercury is more like Jupiter than it is like Ceres.)
Apparently it's not very significant since not only are planets smaller than stars, they are smaller by a pretty large factor.
The mass of Jupiter is about 1/1000 solar masses. Let's say the average mass of these independently floating planets is about 10 times that of Jupiter, and that the average star is about the same as our Sun or less. That would make the mass of an average planet about 1/100 of the average star, so you'd still need planets to outnumber stars by a factor of 100 just to equal the mass of stars. Wikipedia says that visible matter makes up for about 17% of the total matter of the universe, so even if the mass of planets equaled that of stars (which, with the very very rough figures above, would mean a planet-to-star ratio of 100, or something pretty large anyway), there would still be plenty of dark matter to explain.
We know about dark matter not through micro-lensing but because of galactic structure. Galaxies rotate. If all the mass there was in any given galaxy was just what we could see, the centrifugal force would tear apart immediately. The only way we can account for sufficient gravitational attraction to keep stars in their orbits around the galactic center is to assume a lot of mass we can't see--dark matter. Most calculations based on stellar orbits consistently come up with figures for dark matter of over 80% of the matter in the universe. There's more than four times as much dark matter as what we can see. And whatever dark matter is, it apparently is diffuse enough that we don't see it micro-lensing anything, and doesn't otherwise interact with light or other EM radiation, because we find no trace of it in the light that reaches earth from all corners of the universe; therefore it *can't* be ordinary matter as we understand it, because any form of ordinary matter in that quantity would produce detectable occlusion of the light sources behind it. So what is it? Answer that question and win a Nobel.
There was some attempt a while back to assign dark matter to things like this, or free-floating black holes, brown dwarf stars, etc. I.e., somewhat exotic (or not) objects composed of normal matter.
However, in the past maybe 10 years, the constraints for dark matter come much more from cosmological arguments than from observations of the galaxy today. If you're interested, I'd suggest googling WMAP and baryon acoustic oscillations. The basic idea is that we can study the cosmic microwave background, which is the left over radiation as the universe cooled below a critical point some few 100k years after the big bang. In the CMB are embedded small fluctuations like ripples in a pond after you throw rocks in, which are the imprint of pressure waves spreading outward through the primordial plasma. By studying the size and spacing of these ripples, and applying a whole crapton of cool math, you can deduce things like the speed at which those ripples propagate, which is a direct function of both the total matter density and the baryonic (i.e. normal) matter density.
Of course I'm skipping all the details, but the basic result is that, although we first noticed dark matter from observing the motion of galaxies today, it was confirmed to a much better degree by observing the universe in its birth stages, and it's these latter measurements that tell us that dark matter absolutely cannot be due to the behavior of matter and general relativity as we understand it today.
OK, so I've never really understood 'dark matter', but if there's a bunch of stuff floating about that's not stars and only shows up through things like gravitational micro-lensing ... might this cover some of the mass that is dark matter?
Maybe, for galactic dark matter, which is completely unrelated in every respect to dark matter on larger scales, although ignorant people typically use the general term "dark matter" to refer to all types of dark matter indiscriminately, creating enormous confusion in the process.
Galactic dark matter (GDM) is hypothesized as an explanation for the flat rotation curves of spiral galaxies. Based on the visible matter (stars) in a galaxy we can get an estimate of the mass inside a given radius. At sufficiently high radii we see the amount of visible matter dropping off, and expect that the few stars at even larger radii will start to behave like planets orbiting a distant mass with a 1/r**2 fall off in gravitational strength. But we don't see that. Instead more distant stars move as if the amount of matter inside their orbits around the galactic center contained ever more mass as they get further and further away. We can't see any visible matter to account for this, ergo, "dark matter".
One possible candidate for GDM are so-called "MACroscopic Halo Objects" (MACHOS, to contrast them with Weakly Interacting Massive Particles, or WIMPS. Physicists really need to get out more.)
An impediment to the MACHO hypothesis has been that the Initial Mass Function, which describes the probability of an object of mass M condensing out of a primordial cloud of gas and/or dust, was believed to drop off rather steeply at low masses. This observation suggests that it is at least a little higher than previously estimated, although I don't know if that is anywhere near high enough to account for a significant portion of GDM--my sense is not, but it's been a few years since I've paid much attention to this question.
At larger scales we also see anomalous motion of galaxies and galactic clusters relative to the amount of visible matter, and at the very largest scale there is much less visible mass than required to keep the universe in the state of almost-but-not-quite-closed that we see. If these phenomena are caused by an excess of matter at larger scales we know that it is non-baryonic (not made of protons and neutrons) because we have a very good estimate of the density of protons and neutrons in the universe based on primordial nucleosynthesis: the denser the early universe was in protons and neutrons, the more helium would have been created, and given we know the early universe was about 23% helium (there are complex self-consistency checks on this number based on other atomic species) we know there are not enough protons and neutrons to account for the large-scale dark matter (LSDM).
Therefore, we know that LSDM is completely unrelated in every respect to GDM: the problems they solve have different constraints and one requires exotic new physics while the other is relatively mundane. It is deeply unfortunate that people are so incompetent in their use of abstractions that they are chronically unable to distinguish between these two unrelated problems.
Blasphemy is a human right. Blasphemophobia kills.
Free-floating planets are generically expected: Essentially all models for how solar systems like ours (and the others we now know) involve dynamical interactions that would kick out planets at high velocity, leaving them unbound. Astronomers have expected to find these for decades, but have been unable to do so because a planet not warmed by a nearby star gets cold fast (hundreds of thousands to millions of years) and therefore invisible even in the infrared. This result is very important if correct, because gravitational lensing is an emission-insensitive way to find the planets. And yes, IAAA! (ps As for whether they are "spacecraft": I love that idea, but the "people" onboard probably wouldn't give the planet an impulse themselves (way, way, espensivo), they would make use of a free-floater passing by and hitch the ride.)
David W. Hogg -- assoc prof, NYU Physics
Ok, so I actually have an advanced degree in astrophysics. While you do describe the basic observations that lead us to believe dark matter exists, it's not true that galactic dark matter and large scale dark matter are different beasts. True, MAssive Compact Halo Objects (such as rogue black holes, neutron stars, brown dwarfs, etc) (note the correction) were a possible explanation, but we've done observational studies that look for them using microlensing, and although we did find a few, there wasn't nearly enough (i.e. several orders of magnitude less) to explain our galactic rotation curve. WIMPS (such as neutrinos) have been ruled out since they fail to explain the observed large scale galactic structure, and there aren't nearly high enough neutrino counts in neutrino observatories to make them a viable option.
It turns out that, for BOTH galactic rotation curves and large scale darkmatter, you need about 10 times more mass than what we can associate with stars, so the two problems actually do have the same constraints. Therefore, it's very likely that there is some form of matter which only interacts gravitationally (and not electromagnetically: i.e. with light) with normal baryonic matter which has so far been unobserved. (Not surprising, since a lot of our matter detection techniques rely on interactions with light, and besides the required density of this stuff would make it very rare on Solar System scales - it only becomes significant on galactic scale interactions).
On the original article - It's not too surprising that there are lot of free roaming planets, it just indicates that there was a higher degree of fragmentation in molecular clouds than was once thought. However, it would require an insanely HUGE number of them to explain dark matter observations - planets are generally much less massive than stars, somewhere around 10^6 to 10^8 times less massive, so there would have to be 10's of billions times more of them than stars to explain dark matter observations, something that the article does not assert is true.