I have seen the dark universe yawning
Where the black planets roll without aim,
Where they roll in their horror unheeded,
Without knowledge, or lustre, or name.
"There are worlds out there where the sky is burning, where the sea's asleep and the rivers dream, people made of smoke and cities made of song. Somewhere there's danger, somewhere there's injustice and somewhere else the tea is getting cold. Come on, Ace, we've got work to do."
So a question for the knowledgeable on this subject. Over the last few years, we continually keep seeing discoveries large number of intra and extra solar system astronomical bodies that do not radiate at high luminosities or frequencies. Presumably all these add to the mass of galaxies.
How much of the dark matter hypothesis is dependent on observations of rotation of high luminosity bodies? If we assume there are large numbers of these non luminous bodies distributed between stars, does the necessity for postulating exotic dark matter go away?
No, it doesn't help. Brown dwarfs as dark matter was a somewhat mainstream theory in the late 90s under the name MACHOs ("MAssive Compact Halo Objects"), but a seminal 2006 study of colliding galaxy dynamics in the "Bullet Cluster" definitively ruled out that possibility:
The distribution of mass in this cluster shows that dark matter does not obey the same physical laws as baryonic matter. Baryonic matter (including interstellar dust, planets, brown dwarfs and stars) self-interacts through electrostatic friction, transforming kinetic energy into heat. Dark matter doesn't interact with baryonic matter in this way, and passes right through without being slowed down by friction. So whatever it's made out of, it can't be protons and electrons.
No, this was not the conclusion of the Bullet Cluster observation.
In the Bullet Cluster, the dark matter and the clumped baryonic matter (like stars, planets, and putative MACHOs) both pass through the collision, unaffected. (For the dark matter, it's because it's non-interacting. For the clumped baryonic matter, it's because there are no long-range forces and it's vanishingly rare for two stars or planets to hit each other.) It is only the plasma (which is baryonic but not clumped) that is slowed down by the collision, due to the long-range electromagnetic forces between charged particles. Since the plasma constituted the bulk of the baryonic matter in the collision by mass, outweighing the stars, this observation is thought to rule out theories that explain missing mass by modifying gravity at long distances (though this interpretation is disputed). The Bullet Cluster is not evidence against MACHOs because they would have followed the exact same trajectory as non-baryonic dark matter.
From the abstract:
> Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated....we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass
component, but rather approximately traces the distribution of galaxies. [A] spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in
the system is unseen.
MACHOs are generally taken to be ruled out by gravitational microlensing results, not the gravitational (macro)lensing seen in the Bullet Cluster.
Dark matter is going away, regardless, because it is not necessary to explain galaxy rotation curves.
https://www.youtube.com/watch?v=PL0ewiwqoTw
Dark matter was always just a way to fudge what was observed. It is hard to believe most physicists, astronomers and cosmologists still believe in and are still looking for something that never existed.
My understanding is that MOND is essentially dead at this point and that there aren't any other promising alternative theories left on the table.
I enjoyed a podcast on the topic by the highly respected physicist Sean Carroll (along with his guest, MIT physicist Lina Necib). It was very interesting and insightful. They cover in-depth why we are so confident that dark matter is a thing that exists.
Simulation in video I linked is not MOND. Dark Matter is postulated due to observations of galaxies in isolation. BUT galaxies are not gravitationally isolated. They gravitationally affected by all the other galaxies in their neighborhood. The video and simulation is pretty straight forward. Dark Matter is simply unnecessary to explain galaxy rotation curves.
Even if you are correct about the galaxy rotation curves -- which everything I've read and heard leads to me believe is rather unlikely -- those curves are only one of several pieces of evicence that point to the existence of dark matter. The wikipedia page list many of these: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
I only mentioned MOND as it is one of the more widely-known plausible alternatives that have been pursued.
And I and the video creators are only claiming dark matter is not needed to explain galaxy rotation curves. Whatever other spaghetti lumped into the need for dark matter, have at it... but galaxy rotation curves are explained quite neatly looking at two things: 1) dark matter was initially postulated and never reexamined because galaxies were observed as gravitationally isolated and 2) when not unnaturally gravitationally isolating galaxies from their neighborhood group, the rotation curves make precisely exact and perfect sense
https://www.nationalastro.org/news/cool-new-worlds-found-in-... tries to hide it, too, but says, in note 1: The closest of these new discoveries is roughly 23 light-years away from the Sun. Many more of these brown dwarfs are in the 30–60 light-year distance range.
Why do they always reference the nearness to the Sun, rather than just to us? With 8 light minutes between us and our nearest star, and light years to the dwarf stars they are as close to us as the Sun. Your average person is going to read the statement as being near to the Sun almost as if they are close to it but far from us.
Earth tends to move around (literally, or at least elliptically) quite a bit over the course of the year. It’s not much but it is definitely not still.
In the grander scheme of light years the sun is less wobbly in its position.
However, I live 50 feet above sea level, a baseline not known for its short term (or long term) stasis, so I suppose its not exactly as if logic rules the language of waves and stars.
I think the best way to phrase it would be “close to the solar system”, but even then, “close to”, IMO, doesn’t really carry the message for the general public. Neptune is 4 light hours from the sun. 20 light years is 40,000 times that distance, so that’s like saying somebody 40 km away is “close to you”.
I mean, compared to the distance to the moon, yeah - 40km is a rounding error. I don't care if I'm measuring the distance to the moon from my location or 40km away.
And that's 4x less of a difference than your 20 light-year example (moon is ~10,000x further than 40km). Choosing "from us" seems more than precise enough.
If this "average person" is going to get such a false impression from clear and correct information, then maybe there's no use them knowing it anyway? They're obviously not even trying. I know someone who thinks the Milky Way is the same thing as the solar system. That person won't be reading an article about astronomy and it's OK. They have their own life and don't care about it.
Feels like you’re calling me an idiot because, I’ll admit it, that’s exactly what I thought. You’re right in that I care significantly less now than I know the scale of “close” is not actually close in my terms. But it made it sound like a more impressive discovery than I perceive it to be. So, what they did is astronomical linkbaiting.
Sorry, I was going by the GP's mentioning light years. I see now that the article doesn't say that at the top, just "near the sun" which I agree is weird and confusing.
I assumed “near the sun” the meant between us and the sun, which would have blown my mind having taken just a few undergrad courses in astronomy and physics.
As the wikipedia article states, the WISE survey[1] results would have turned up Nemesis or anything like it, and did not. Currently most likely theory for the improbable distribution of objects beyond the Kuiper Cliff is Planet Nine.[2]
From
8 parsec to 70 parsec,
so 28 to 250 light years
or 240 trillion kms to 2170 trillion kms
away (-ish)
Many are comparatively cold. While most seem to be 400 Celsius plus, one (couldn't find in the article, but mentioned elsewhere) seemed to be -10 Celsius.
Roughly (waving my hands and talking vaguely here) about 10% plus minus in distance and temperature.
So that's actually further away than the closest star systems. Got freaked out for a second, because having hard-to-see massive objects near your sun system is scary!
The line between small brown dwarfs and large planets is blurry. A brown dwarf twice the size of Jupiter and twelve times its mass could probably get inside our Oort cloud and have evaded detection.
In the scifi short story "a pail of air", the Earth is yanked away from the Sun by a dark star passing through the solar system. The Earth cools until all of the atmospheric gasses turn into solids and fall to the ground as layers of ultra-cold snow.
It's one of the pieces of fiction that left a deep impression on me. Harsh winter days still remind me of it.
When I was little I read a Jules vernes story about a big asteroid touching briefly earth and yanking away some water and an island. Presumably this is only perceived by the islanders as a big earthquake and it takes some time for them to realise that they are not on the earth anymore. Next year three is a close pass near earth again and they manage to return to earth in a balloon :)
The fact is that 250 light-years is not "near" the sun. Unless you're an astronomer, there's no practical difference between something that is 2 light years or 2,000,000 light years away. Both distances are equally unattainable. Saying that something is near "the sun" (as opposed to a more generic area like "the solar system") makes it sound like it's much closer than it actually is. Imagine if the article said that the stars were near "Dallas Texas". It would be equally true (±0.001%), but IMO the extra specificity is misleading
Exactly. They are not specifically near 'our sun'. They are near our system. The title (because of the specificity), seems to say that they are in our solar system, or so close that you could consider it like that
A primordial black hole of 1000 Earth masses is only 17 meters wide. It's effectively impossible to see, with a event horizon temperature of 0.02 millikelvin. If it was extragalactic, it'd pass through our solar system at a pretty good clip. It could certainly do unfortunate things to Earth's orbit. A normal stellar mass black hole would be even worse.
The other day I saw a CCTV video that had been turned into a bit of a social media sensation, in which an elderly fellow is walking his dog when out of nowhere, a random house cat falls from well above and lands on his head, knocking the man flat unconscious. The dog continues on about his exploring for a few seconds more before casually sniffing his owner to see what's up with the guy. It then just as casually moves its attention over to the cat, which is apparently totally fine despite its fall and rough landing (so typical of cats), and starts to play with it over in a corner of the video. The dog was wearing little red boots on its front paws by the way, just in case everything else about the video wasn't absurd enough already...
My point? That even this was a much better candidate for a far-fetched event in 2020 than this pandemic ever could be. Scientists, academics, reporters, policy makers and even some more astute or honest politicians have been predicting it for literally years as something that was probably just around the corner.
The response of various countries mostly follows pre-pandemic approach of the respective governments to science, public services and making hard decisions. Personally, the only major surprise here is the level of defending the stock market.
Yesterdays science fiction more often than not is today's science. As long as you leave time travel, wormholes, fusion and aliens out of it has surprising applicability.
The facts say that such an event has an extremely low probability of happening, as our solar system is still intact after 4.5 billion years and solar systems remain around nearly all stars we look at, so the probability of this happening is so low as to not worry about it.
I actually agree that the probability of such an event is very low on the scale of a human lifetime or even the lifetime of our species, and I'd never worry about it.
I had interpreted your "why would it be scary" question as "what kind of bad things would happen if a brown dwarf did get close?", but I guess it was instead a rethorical question implying something about low probability. Hence the confusion.
That said, there is a problematic selection bias in your statement about our solar system. Only stable systems can produce observers, so of course we find ourselves in a stable system. For now ;-)
Yes, I meant "why are you scared?" It's a huge problem I've encountered where a very large percentage of the population get freaked out about things like black holes, rogue planets, gamma ray bursts, the sun eventually going nova, etc. As someone whose background is astrophysics and planetary science, I find it totally bizarre.
Like, we just found out the universe is a way, way bigger place--full of planets between the stars. Maybe humans will visit these worlds someday as we planet-hop between the stars. What civilization could we make near these cold, but resource-rich worlds? What if they had tidally-heated moons with liquid oceans... what sort of life would evolve without the energy of a star?
But no, people's first reaction is "OMG is this going to kill me?!" I've even had someone get angry at me for bringing up the topic (that the sun will go nova billions of years from now) and making them depressed. I just don't get it.
> That said, there is a problematic selection bias in your statement about our solar system. Only stable systems can produce observers, so of course we find ourselves in a stable system. For now ;-)
That's why I mentioned other exosolar systems. So far as we can see, basically every stellar object that we look at has stable planetary system. We mostly find planets by periodic transit events, which requires the plane of formation to be aligned with us. You can calculate the statistical chance that this is the case, and sure enough when we look we find roughly that percentage of stable planetary systems. If there were rogue planets or primordial black holes out there likely to encounter us, then those other systems would be just as vulnerable and we would expect to see fewer percentage of visible stable planetary systems than predicted. That is not the case.
We would also expect consistent groups of long-period comets from the interactions of these passing objects with the Kuiper belt and Oort cloud, but there is no consistent grouping.
We would expect a history of random heavy bombardment periods in the crater history of the moon, mars, mercury, and other datable planetary bodies in the solar system. Instead we see just one period, the late heavy bombardment which coincides with the synchronization of the orbits of Jupiter and Saturn in the late stages of solar system formation.
Etc. etc. When I first saw the headline I thought "wait a minute, that cannot be right." And sure enough, it's not. There aren't 100's of rogue planets in our particular stellar neighborhood, but rather scattered around the nearby region.
If a brown dwarf scatters into the solar system it might dramatically shift orbits potentially making earth uninhabitable. To say nothing of a direct hit on anything, even hitting the sun might strip atmosphere from earth, hitting any planet would create world ending debris fields
> Could be an object at high speed, waiting for the right gravity assist directly into us?
An object larger than Jupiter doesn't change its course because of chance encounters with other random small bodies.
> Could be a strange object with capability to disrupt the sun somehow?
This doesn't make physical sense.
> Why would an object hitherto unbeknownst to us and close to the sun NOT be scary?
Because it's had no observable effect on us for the last 4.5 billion years of our solar system's existence, and we've postulated nothing that would change this stable dynamical relationship?
Then I guess the morons with careers in astrophysics are investigating it for no reason then.
Beyond your wildly unfounded and I daresay naive assumption that the objects have had no effect on the solar system, perhaps they need to hear YOUR postulations.
If only they were reading all of your grayed out comments, I see your dismissive and arrogant condescending remarks are leading to such fruitful insights and conversations. /s
New things are interesting, and astrophysicists like investigating interesting things. If these brown dwarfs posed the slightest threat to us they'd be even more interesting than they already are.
Sure, I guess it's just irksome to assume that they aren't dangerous PRIOR to investigation. I mean, you wouldn't assume a random plant in the jungle is safe to eat, right?
There's a lot of safe objects in space. There's a lot of safe plants in the jungle. Still, truth must be uncovered day by day.
This somehow brings up the fact that we say "innocent until proven guilty" but we don't really treat people like that, see: arraignment amounts/remand, police interrogation, the fact that you both have to know your rights and have the money to at least get decent representation for most things.
Because it's had no observable effect on us for the last 4.5 billion years of our solar system's existence, and we've postulated nothing that would change this stable dynamical relationship?
Okay, but speaking strictly in the human timescales of the next few dozen or few hundred generations of our species, a complex interaction of celestial phenomena that hasn't led to any known cataclysms during the last 4 billion+ years could possibly change in a way that causes it to do so, but the chances of that happening specifically now or in the next few hundred generations of human development are vanishingly small.
It's like you climbing a volcano that has been dormant for 1000 years, and it suddenly, explosively erupting right while you're walking towards the peak. Could happen out of the blue right during those few hours, sure, but it really, really probably won't.
The solar system is actually not stable in the long term. Early planetary development was violent and chaotic, and the planets shifted around a lot more than you might think.
We can't project orbits out indefinitely, either; multiple studies show that our ability to go out more than a few hundred million years are suspect and very dependent on starting conditions, which can obviously be easily perturbed by unknown bodies. Since we get star-sized approaches at Oort Cloud distances on the order of every few million years, that probably puts an upper bound on things.
However the rate at which such encounters happen varies greatly. We are in an orbit bobs up and down from the galactic ecliptic with a period of roughly 60 million years. There are more encounters near the ecliptic, with very few encounters when we are above or below. We cross the ecliptic about every 30 million years, and last did so about 3 million years ago. So we are still in the dangerous period.
It is not entirely a coincidence that the dinosaurs were wiped out 66 million years ago, during another relatively dangerous period.
If it were in our solar system then it would be in a gravitationally stable arrangement--because if it weren't it would have already thrown things out of whack in the last 4.5 billion years.
I had to look it up, because I'm curious too. It looks like volunteers are given images from NASA's Wide-field Infrared Survey Explorer (WISE) telescope. The volunteers then just look for moving objects. https://www.zooniverse.org/projects/marckuchner/backyard-wor...
I was unable to find a table matching wavelengths to visible temperatures in a bit of searching. Closest I could find is the chromaticity diagram on Wikipedia's Black-body Radiation article, which puts 1000K around 0.7nm red light.
The WISE article lists 22μm as its longest wavelength, though that might no longer be attainable since WISE is out of coolant. If one can find out what blackbody temperature has its peak at 22μm, then we could maybe see how close to the edge of WISE's temperature range the 400C (673K) and -10C (263K) dwarves are.
Looks like 20μm is a bit below 200K, so if I'm interpreting this right, WISE should have had no temperature-related problem (leaving aside brightness) spotting a -10C brown dwarf.
For anyone missing how little radiation that is, radiation is proportional to the 4th power of the temperature in Kelvin.
200 K is a bit below (water) freezing. 400 K is just over boiling. The 400 K object emits 16 times (2 ^ 4) more radiation than the same object at 200 K.
From my understanding they took 2 pictures of the same spot at different times and then had volunteers (“citizen scientists”) look at those pictures to see if anything moved. You can see some examples of the kinds of pictures in the paper linked in another comment above.
Really minor correction, but 7.1±1.4 is the lower bound. 1 parsec is a tad over 3 lightyears, so it's a somewhat significant difference when the distances are that small.
I find the phrasing odd as well. Why specify near our sun as opposed to near us? If it's specifically near our sun as opposed to near us that would imply something closer to the sun than us.
Space.com likes to write those kind of titles. However, the observable universe is ~93 billion lights years[0] across so 20-60 light years is relatively close.
My first reaction when I saw the headline was NEAR THE SUN. As in, Mercury near. As in 'near-sun-objects'.
TA mentions no distances. Even 1 to 10 LY away (1/4 to twice as far as Proxima) ... is not so 'near'. If we use Pluto's orbital diameter as that of the solar system, then it's only about .001 light-years.
'Backyard'? If my backyard is 100 feet long, a thousand of those is 19 miles.
Credit where it's due, "Backyard Worlds" is one of a large number of "citizen science" projects hosted at Zooniverse[1]. The specific project is at [2], but the Zooniverse platform hosts many others affording pleasant hours of internet contribution.
What’s the connection between the brown dwarfs being cold and being exoplanets? Are planets definitionally not hot, or does it tell us something about how the dwarfs formed?
The definition of a brown dwarf is enough mass to cause nuclear fusion of deuterium (roughly 13x Jupiter's mass) but not enough for hydrogen-1 (roughly 75x Jupiter or 0.08x the Sun.)
Planets by definition are anything that doesn't meet the deuterium threshold. Stars meet the hydrogen threshold. "Cold" in this context means relative to hydrogen-fusing stars. It's referring to the object's own energy production, not like measuring the surface temperature of a planet illuminated by a star.
That we are finding "cold" brown dwarfs now is observational bias. We already found the hotter ones because they're more luminous and detectable. "Near the sun" (up to about 250 light-years here) is also observational bias, we just can't detect cold ones any farther.
One of the proposed dark matter explanations is "massive astrophysical compact halo objects" (MACHOs). Brown dwarfs are among some of the possible MACHO candidates.
One place where brown dwarf (BD) and exoplanet science overlap is atmospheric characterization by spectral methods. If you get reasonably well-resolved spectra from the BD, you can invert to obtain information about its atmosphere's composition and density/temperature profile.
The BD is going to be more emissive than an exoplanet (and a BD has no light contamination from a nearby host star) so the BD is not an exact analog, but BDs provide a workshop for development of inversion approaches.
I think the brown dwarf still fuse deuterium but then the question is - how can it be so cold? And if it is so cold - is it a planet. But then why is it so (relatively) hot? Could be that the new discovered object is (thus far) a missing piece on a spectrum between brown dwarfs and gas giants?
One of the difficulties in astronomy is determining the age of astronomical objects.
Brown dwarfs and small red dwarfs have about the same temperature in the early stages of their formation. The main difference between the two is the drop in temperature over time.
A super cool brown dwarf might just be an old brown dwarf. The (arbitrary) line between planemos and brown dwarfs can be drawn via mass (e.g. no matter the temperature, if it's too small for deuterium fusion, it's not a brown dwarf) and composition (e.g. via testing for lithium), though both criteria are not perfect.
A cool brown dwarf can basically just be very old.
The paper Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project [1]:
> We present Spitzer follow-up imaging of 95 candidate extremely cold brown dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project, which uses visually perceived motion in multi-epoch WISE images to identify previously unrecognized substellar neighbors to the Sun.
Perhaps a floating colony like those proposed for Venus? There might be a Goldilocks zone in the atmosphere. If not then there may still be a comfortable location in near orbit where the heat would be tolerable. It’s interesting to think of ways to overcome an extreme gravity environment though, perhaps suspension in a liquid would counteract some of the effects?
> Some of these weird worlds are even relatively close to Earth's temperature and could be cool enough to have water clouds in their atmospheres, according to the statement.
Should be noted that this kind of science in particular will be completely destroyed from satellite constellation launches from the likes of SpaceX and Blue Origin.
With the amount of satellites being launched, it will be impossible not to have several in frame, especially over a long exposure, and they will swamp out pretty much any signal from space with their brightness.
You don’t need long exposure now that we have digital cameras, it’s better to stack many short exposures. It’s trivial to throw out the outlier values which removes the satellite trails, and other sensor noise.
Images with prominent streaks are being deliberately processed to only keep the bright outliers and delete the clean frames.
