Which stars can see Earth as a transiting exoplanet?

https://academic.oup.com/mnrasl/article/499/1/L111/5931805

by colinprince • 5 years ago

138 46 5 years ago

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lmilcin• 5 years ago

The best way to search for extraterrestial life I heard is a swarm of drones placed in such a distance from the Sun that it acts as a gravitational lens to greatly magnify any planet around any star in even quite large distance from our system.

I think realistic resolutions in kilometers per pixel are theoretically possible even with existing technology (assuming we can build the swarm and position it in the right place quite far from the Sun and then maintain communication).

The disadvantage is you need many separate fleets of drones to map more than one target.

shoto_io• 5 years ago

Yes. This video explains it rather nicely: https://www.youtube.com/watch?v=NQFqDKRAROI

northwest_nfts• 5 years ago

Yeah, that sounds similar to the idea of the Terrascope. I think the positioning of drones for the sun might be so wide to be infeasible for the next century. (Like, the drones would have to be in the Kuiper belt)

I haven't watched this YT video recently, but I think he also touches on a Sol-scope and a Jupiter-scope https://www.youtube.com/watch?v=jgOTZe07eHA

pavlov• 5 years ago

Would Sol look obviously inhabited on the radio spectrum?

This is a common sci-fi trope (“Contact”), but I have no idea whether it’s actually true that our radio emissions would be trivially detectable tens of light years away.

wyldfire• 5 years ago

What if they had an array similar to the one used in "Contact" to receive the response?

fermuch• 5 years ago

Our radio waves have traveled for about 200 light years. Proxima centauri, our closest star, is located at 4246 Light years from us.

Very, very unlikely that someone can hear us, even if they tried. Space is huge.

gpderetta• 5 years ago

I think your Alpha Centauri distance is off by 3 orders of magnitude :)

Edit: I think there are thousands of stars within 200ly of Earth.

ajakate• 5 years ago

Proxima centauri is actually ~4 light years away. Space is really big, but there are a handful of stars in relative vicinity of us (under 250 ly)

https://en.wikipedia.org/wiki/Proxima_Centauri

http://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/LocalGroup/Ba...

ortusdux• 5 years ago

The inverse square law really dilutes out most signals - https://www.quora.com/How-far-do-radio-signals-travel-into-s...

ThomasWinwood• 5 years ago

As I understand it the amount of time we were tossing radio into space was quite brief, and more modern radio traffic is directed at the ground (since after all if you're leaking signals into space you're wasting power).

One of the papers cited in the OP argues that a good indicator that there's something interesting about a planet is the combination of molecular oxygen and methane in its atmosphere - the former is a strong oxidiser, while the latter is a reducing gas, so there's some dynamic system producing free methane in the atmosphere.

mandevil• 5 years ago

From the 1960's to the present, the Ballistic Missile Early Warning System radars have been broadcasting enormous amounts of coherent power on specific, non-natural frequencies into outer space: if we were to detect a signal like that we'd almost certainly determine it was artificial.

You can see the list of stars which would have recently started seeing the first glimpses of the BMEWS Line radars here: https://en.wikipedia.org/wiki/List_of_star_systems_within_55...

Obviously many other radars (DEW, Hen House, etc.) are doing the same thing, but BMEWS has continuous operation and power on its side.

rootusrootus• 5 years ago

I hope that little forest of stars is not just dark, but actually empty.

zardo• 5 years ago

How far away will the signal be detectable above the background noise?

giantrobot• 5 years ago

In theory yes but in reality Earth rotates and orbits the Sun. Any particular observer outside our solar system has a very low probability of catching any one of those beams at the moment it's photons are reaching them.

There's also little chance of those beams will ever hit the same observer. The inverse square law also means that a potential observer would need to be in a relatively small radius from our system.

rimliu• 5 years ago

And those signals have travelled 60 light years. Our Galaxy is 100 000 ly accross…

ChuckMcM2• 5 years ago

I would guess, "probably not" for a variety of reasons.

First, the ionosphere bounces a lot of radio traffic right back down to the planet, amateur radio operators (like me) use that to talk to stations farther away on planet but it doesn't go into space.

Second the frequencies that do travel through the ionosphere are typically aimed at satellites (geosynchronous or lower) and are scaled to have a level of power that gets there reliably but without an overly large margin for error.

Third, the Sun is a very "noisy" source of RF energy and someone looking this way at us would also be looking at the Sun as well (if you're looking at transits). In terms of radio frequencies that they could perhaps analyze it would require a lot of filtering.

The analysis we've done on transiting planets so far is based on spectroscopy using the star as a huge light source and looking for how that light is filtered by the 'edges' of the planet. (so through the atmosphere of the planet).

If they are doing that, then the change in atmosphere composition over the last 1000 years seems like it would be detectable.

nawgz2• 5 years ago

the change in atmosphere composition over the last 1000 years seems like it would be detectable

Interesting to think what conclusions would be drawn in these scenarios. Obviously atmospheric composition has drastically changed over the eons - I wonder what sort of civilization could detect these kind of longterm changes, and if they wouldn't have better observational capabilities than what we assume here given they have persistent knowledge over "long" timespans

dvtrn• 5 years ago

I would also imagine that in addition to those three, regular old-fashioned attenuation would degrade the signal at significant enough distances as well, no? Or is that not an issue in the vacuum of space?

xwdv• 5 years ago

What kind of things do you talk about with radio stations so far away?

samsari• 5 years ago

I've also read that modern digital communication signals would appear very similar to normal random RF noise.

koheripbal• 5 years ago

it's important to remember that the Earth emits 100x the radio wave energy than the sun.

At some point between 50 and 100 light years the Omni directional radio waves blend with the background radiation.

I have seen projections of "plausibly easy" detection up to 50 light years. They difference between the number of stars that are 50 light years vs 40 light years is about 2x, so the calculation of how many stars are in range is actually very sensitive to emission range. There are 64 stars 50 ly out, and almost 500 up to 100ly out.

That's still not that many though, given the 500 billion stars in our galaxy. It seems only plausible that an ET intelligence would detect us if they have seeded the galaxy with probes - in which case their would probably be one listening in our own system anyway.

DamnInteresting• 5 years ago

If you'll excuse the self-link, I have written about this at length: https://www.damninteresting.com/space-radio-more-static-less...

In short, even our largest, most sensitive radio telescopes are far too small to detect omnidirectional (e.g., FM radio) leakage from Sol's nearest celestial neighbor. Unless an exoplanet directs a coherent radio beam at Earth, we'd need an antenna larger than the planet to detect a signal amid the noise.

oh_sigh• 5 years ago

To put it generally: How far could we be from each other and still detect ourselves?

credit_guy• 5 years ago

No, our radio emissions would not be trivially detectable. In fact they would be phenomenally difficult to detect even from our nearest stellar neighbor, Proxima Centauri. If you take the most powerful BBC antenna [1], then a James Webb-sized telescope placed near Proxima Centauri, and pointing straight at the Earth would receive less than 5 photons per second from that antenna.

Here's the calculation, in case you're curious: one light year is about 10^16 meters; a sphere of radius 4.25 ly has an area of 2 10^34 m2. The BBC antenna emits in the 200 kHz band, so a single photon has an energy of (plank const) x (frequency) = (6.6 10^-34) x (200 10^3) = 1.3 10^-28. The antenna has a power of 500 kW, so that translates into 3.8 10^33 photons per second. That means at Proxima's distance each square meter will be crossed by a photon about once every 5.4 seconds. The James Webb telescope has a receiving area of 25 m2, so you end up with a bit less than 5 photons per second.

[1] https://en.wikipedia.org/wiki/Droitwich_Transmitting_Station

belter• 5 years ago

They are trying to communicate with us with Quantum Entanglement. Until our physics progresses, so as to understand that Quantum Entanglement does not limit you to transfer of information at the speed of light, we are considered a level IV Civilization.

The current agreement is that level IV Civilizations should stay "Uncontacted" :

https://en.wikipedia.org/wiki/Uncontacted_peoples

f00zz• 5 years ago

Crazy to think that we have catalogued thousands of exoplanets, even though we basically can only detect them if they pass in front of their stars along our line of sight. Space is big, but also pretty crowded.

Aerroon• 5 years ago

I think it's more that we cab tease out a lot of information from very little data by just making some assumptions (physics). This is also why it's so difficult (impossible?) to anonymize personal data - very little of it can lead to understanding who you are.

It is remarkable what astronomers/physicists have achieved though.

holoduke• 5 years ago

And probably there are more roque planets than planets with a star. Those are even harder to detect.

bulatb• 5 years ago

Event Horizon recently interviewed Lisa Kaltenegger (coauthor here) and Jackie Faherty about a similar paper they wrote: "Past, present and future stars that can see Earth as a transiting exoplanet."

https://www.youtube.com/watch?v=W5CV6exdqcg&t=1m56s

dekhn• 5 years ago

People who are interested in learning more about transmitting information between solar systems should watch this wonderful talk: https://www.youtube.com/watch?v=sImBlq542TQ the author is a professor at harvard (known for writing 'The Art of Electronics' in addition to searching for extraterristrials) and put a lot of good thought into light transfer between solar systems.

TL;DR a powerful enough laser that is well collimated would work.

Kattywumpus• 5 years ago

Aliens Can Watch ‘I Love Lucy’: https://www.lscheffer.com/tv.pdf

seventytwo• 5 years ago

What a fantastic question! I hope to see more of this kind of thing

Which stars can see Earth as a transiting exoplanet?

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