Volunteers spot almost 100 cold brown dwarfs near our sun
space.com
space.com
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?
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.
Here's a link to it: https://www.preposterousuniverse.com/podcast/2020/05/11/96-l...
I only mentioned MOND as it is one of the more widely-known plausible alternatives that have been pursued.
https://arxiv.org/abs/astro-ph/0608407
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.
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.
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.
It would be interesting to know if a theoretical smallest coolest brown dwarf would be detectable with the current technology.
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.
Could be a strange object with capability to disrupt the sun somehow?
Why would an object hitherto unbeknownst to us and close to the sun NOT be scary?
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?
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
Feel better soon
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.
That's all I was meaning to convey.
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.
It's one of the pieces of fiction that left a deep impression on me. Harsh winter days still remind me of it.
It reads as if it is either between us and the sun, or close enough that you'd see it roughly up to the same distance we are from the sun.
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.
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 ;-)
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.
Very unlikely, of course. But not fictional.
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.
https://en.m.wikipedia.org/wiki/Wide-field_Infrared_Survey_E...
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Edit: found it: https://commons.m.wikimedia.org/wiki/File:Blackbody_peak_wav...
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.
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.
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.
EDIT: corrected my correction
[1]https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Exp... [2]https://en.wikipedia.org/wiki/Planet_Nine
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.
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.
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.
And Fahrenheit on a science website? Use kelvin or at least Celsius not Fahrenheit.
What an annoying article.
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.
[2] https://www.zooniverse.org/projects/marckuchner/backyard-wor...
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.
Even in our own solar system we don't know whats out in our Kuiper Belt. Probably nothing too big or we'd be able to measure the gravity effect.
But in the Oort cloud, we could have multiple Pluto sized things out there and have no idea.
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 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.
> 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.
"Brown dwarf" is an object bigger than a usual planet, but not big enough to spark and sustain nuclear fusion in the core.
"cold" means "not hot enough to shine bright like a star".
Jupiter seems to have sufficiently dynamic atmosphere. Not sure about cold brown dwarfs...
https://phys.org/news/2020-05-costly-collateral-elonmusk-sta...
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.
Once they're in Earth shade, they have no astronomy impact.