High Contrast Imaging of the Exoplanet HIP 65426 b from 2-16 μm
arxiv.org
arxiv.org
https://i.imgur.com/KlgEVze.png
Take a look at this sequence:
https://i.imgur.com/STcNkIP.png
They started with the image at left, which represents a combination of both the planet and the star. Clearly, the star is so much brighter than the reflection of starlight off the planet that it blows out the whole image. But the star is pretty consistent, and definitely circular, so they could take a reference image and a rotated image and erase all the pixels that look like they expect the star to look. The difference between a normal star and the image they actually took is the planet-shaped hole left after subtracting one from the other.
>Comments: 35 pages, 16 figures, 3 tables, 1 wonderful telescope; Submitted to AAS Journals
You can get some maps at e.g. <https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brow...> and more if you explore through the cited papers.
There are 10 known stars or star-systems within approximately 10 light-years of our local star (Proxima Centauri, the Alpha Centauri triple, Barnard's star, Luhman 16 in Vela, W0855 in Hydra, Wolf 359 in Leo, Gilese 411 in Ursa Major, the Sirius binary, and the UL Ceti / BV Ceti binary and right on the edge of our 10 light-year bubble V1216 Sagittari). There are 93 within 20 light-years. These counts include multi-star systems.
I write "known" because there may be dim stellar objects we haven't found yet. The double in Vela are both dim brown dwarfs, much smaller than the sun, and were only discovered in the last decade or so, despite being less than seven light-years away. Brown dwarfs are hard to spot. Red dwarfs become similarly hard to spot at a distance of only 20 light-years.
Without descending into power-law discussions (although see <https://news.ycombinator.com/item?id=32613762> if you are curious and want to think about the density of dim dwarfs vs bright naked-eye stars), we can just accept that the density of stars within 100 light-years is likely very similar to that within 10 light-years and within 20 light-years.
The volume of a sphere goes 4/3 * pi * r^3. A 20 light-year (ly) sphere has 8 times the volume of a 10 ly sphere. We have roughly 10 stars vs roughly 100 stars, so that tracks roughly. Extrapolating and taking only orders-of-magnitude estimates of star counts, a 100 ly sphere has 125 times the volume of a 20 ly sphere, so we would expect the star-count to rise from roughly 100 to roughly 10000 stars. That's much less than half of the almost sixty-thousand number that your comment's parent suggests.
To multiply the rough estimate based on the 10-ly and 20-ly count by between about four and six to match the "59,722" figure we would have to introduce one or more screening mechanisms or properties to hide so many more stars. Properties include intrinsic dimness and low mass. If we require that stars are self-luminous that means they must be massive enough for nuclear fusion in their cores, so at the lowest end that means brown dwarfs like the ones mentioned a couple paragraphs above, somewhat heavier and brighter red dwarfs, and so forth. Screening properties would involve arranging these unseen stars in such a way that they eclipse one another, are hidden behind brighter dust, and have low proper motion about the "local standard of rest" average motion stars moving in our part of the Milky Way. Not only do we have a sudden jump in the volume-to-mass relationship compared to 10 and 20 light-years, we would also have to cut off the volume-to-mass relationship not too far outside the 100 light year bubble, or dramatically change the shape of our own galaxy compared to that of spiral galaxies, and our understanding of the proper motion of the Andromeda galaxy towards us. We also have to deal with the results from gravitational micro-lensing surveys (e.g. from MACHO hunting), and the study of the fast orbits of stars in the Milky Way's central parsec, neither of which supports large numbers of dim-but-considerably-bigger-than-Jupiter masses lurking in our galaxy.
If we go the other direction and start with the evidence that we are in a spiral arm of a galaxy like Andromeda, and that the "thin disc" stellar density is about 0.004 per cubic light year (<https://en.wikipedia.org/wiki/Stellar_density> and the recent <https://arxiv.org/abs/2207.03492> which maps comparable-scale variations in the stellar density in the Milky Way's thin disc), then in a 100-light-year sphere having a volume of about 4.2 million cubic light years we end up expecting a little fewer than seventeen thousand stars.
Consequently, unless there's something almost uniquely special happening around 30-100 light-years from here, a much better estimate for the star-count within 100 light years is twelve thousand plus or minus five thousand. Note that this is a much less precise figure than the one given upthread.
“thanks to Gaia EDR3, the solar neighbourhood has been mapped with great precision out to 100 pc (326 light years).”
https://www.cosmos.esa.int/web/gaia/edr3-gcns
The page links to the paper, the catalogue, notable figures and a “fly-through”.
There's also a few hundred[0] G-type stars within 100ly of Earth, if we include F and K types we've got thousands of stars within 100ly. We're just on the cusp of observatories that will be able to get spectra from terrestrial planets in close (<5 AU) orbits around their host stars. So a civilization within 200ly of us just slightly more advanced would be able to not just detect Earth, life signs on it, but also have at least a guess there's a technological civilization here.
Those don't say 'dead' or 'stable'.
As long as our theoretical aliens are aware of photosynthesis, which can use a number of pigment chemicals, they would likely settle on photosynthesis. The longer they watch Earth they'll see seasonal variations in CO2/O2 levels.
That's relatively frightening.
Interstellar shipping rates are astronomical (pun shamelessly intended). Even sending a teeny tiny package to the next solar system over costs a lot and takes a long time. The fastest it's ever going to go is the speed of light.
That fact we haven't been obliterated by an alien invasion fleet tells us that there's probably not a hyper advanced alien species that is big mad about us within 100ly. Thats the threshold where a hyper advanced alien civilization could have seen the Industrial Revolution starting, deduced there's a technological civilization here, developed a big mad about the fact, and launched a destruct-o fleet or super weapon at us with hyper advanced speed of light propulsion that is so advanced the trip is practical in terms of expense (practicality factor).
There's a "probability of mean hyper advanced aliens" value that's like the Lindy Effect. The longer we go without being obliterated by an alien space fleet the further away we can assume mean hyper advanced aliens might live. For mean aliens that are less advanced the less we need to worry about them because the practicality factor of their propulsion system is lower.
An alien invasion fleet sent here at at Parker Solar Probe velocities (0.064c at closest approach to the Sun) are less worrying because the likelihood of them arriving at a point where we personally need to worry about them is pretty low.
For instance our practicality factor for building an interstellar invasion fleet is basically zero. Even building a von Neumann probe has a so close to zero it might as well be zero practicality factor. I'd posit that there's a minimum practicality factor a civilization needs to go around destroying other ones. If your best propulsion is below that threshold you're just not going to ever build an invasion fleet. You might build interstellar probes but an invasion fleet is impractical.
So I'm not worried about the hyper advanced aliens watching me shower. And I'm also not worried about the less advanced ones either. Space is really big and traveling between stars is really hard. If my existence made the hyper advanced ones so upset they'd come blow me up there's not a lot I could do about it anyways.
Why do you say this? Could you expand on it? It seems to me like the cost of these sentient, self-replicating factories would be marginal.
I'm worried about them sending a von Neumann probe that constructs a series of asteroid deflection ships that would put impactors on a direct collision course with us.
Or some smart, self-replicating nanobot that can out compete our biological systems for energy while being metabolically undigestable.
Or some kind of energy weapon.
I was saying our ability, as in Earth today, has a practicality factor of zero for building even a von Neumann probe. That's not to say we won't be able to in the future but today the practicality factor is zero.
Take for instance the Parker Solar Probe at its fastest will hit about 193km/s thanks to slingshots around the Sun. It could reach Alpha Centauri in about 8000 years. If we could build killer nanobots we could put a bucket of them in such a probe and launch it at alien planets we didn't like.
Unfortunately for our killer nanobots there's no way to slow them down. If they impacted anything in the system they'd be vaporized. Whatever method we want to use to slow the probe down to some practical speed for a killer nanobot attack, we'd need to be far more advanced than we are today. We'd need extremely accurate maps of the target system, some sort of long-lived power system to run some kind of as-yet undeveloped ion drive to steer/slow the probe, and of course a way to build killer nanobots and a probe that could run for thousands of years flying through space.
So the practicality factor for us building killer nanobot probes is effectively zero.
As for your other points, I'm not worried about those things since they would be undetectable until it was too late. There's a lot more immediate concerns that are far more pressing and likely than any of those.