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SETI In Reverse: Who Could Know About Life On Earth?

EnolaGaia

I knew the job was dangerous when I took it ...
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There's been a lot of discussion and speculation about our ability to detect likely sites for extraterrestrial life and / or alien civilizations. In a new study, two astronomers surveyed our expanding data on exoplanets to do something like SETI in reverse - i.e., identify which exoplanets could represent vantage points for aliens who might detect and / or monitor us.
Alien Planets Around 1,000 Nearby Stars Could Be Looking Straight Back at Earth

Astronomers are working hard to catalogue all of the exoplanets visible from Earth, but now two researchers have turned the idea around, to look at which exoplanets are getting a good view of Earth in return.

It turns out there are 1,004 (and counting) main sequence stars, similar to the Sun, with orbiting Earth-like planets that probably have an opportunity to detect chemical traces of life on our own planet. If there's anyone up there, they can see us.

These stars are all within 326 light-years (100 parsecs) of Earth, with the study focusing on the closest exoplanets first.

Data from NASA's Transiting Exoplanet Survey Satellite (TESS) star catalogue and the Gaia star map was used to make the calculations, and over time the star systems that can view Earth will change.

"If observers were out there searching, they would be able to see signs of a biosphere in the atmosphere of our Pale Blue Dot," says astronomer Lisa Kaltenegger, from Cornell University. "And we can even see some of the brightest of these stars in our night sky without binoculars or telescopes." ...

FULL STORY: https://www.sciencealert.com/plenty-of-exoplanets-could-be-looking-straight-back-at-us-too
 
Here are the bibliographic details and abstract for the published report on this study. The full report is accessible at the link below.

Which stars can see Earth as a transiting exoplanet?
L Kaltenegger, J Pepper
Monthly Notices of the Royal Astronomical Society: Letters, Volume 499, Issue 1, November 2020, Pages L111–L115
https://doi.org/10.1093/mnrasl/slaa161
Published: 20 October 2020

ABSTRACT
Transit observations have found the majority of exoplanets to date. Also spectroscopic observations of transits and eclipses are the most commonly used tool to characterize exoplanet atmospheres and will be used in the search for life. However, an exoplanet’s orbit must be aligned with our line of sight to observe a transit. Here, we ask, from which stellar vantage points would a distant observer be able to search for life on Earth in the same way? We use the TESS Input Catalog and data from Gaia DR2 to identify the closest stars that could see Earth as a transiting exoplanet: We identify 1004 main-sequence stars within 100 parsecs, of which 508 guarantee a minimum 10-h long observation of Earth’s transit. Our star list consists of about 77 percent M-type, 12 percent K-type, 6 percent G-type, 4 percent F-type stars, and 1 percent A-type stars close to the ecliptic. SETI searches like the Breakthrough Listen Initiative are already focusing on this part of the sky. Our catalogue now provides a target list for this search. As part of the extended mission, NASA’s TESS will also search for transiting planets in the ecliptic to find planets that could already have found life on our transiting Earth .

FULL REPORT:
https://academic.oup.com/mnrasl/article/499/1/L111/5931805
 
The idea is getting more publicity:

"In game theory there are a class of games known as coordination games involving two players who have to cooperate to win but who cannot communicate with each other. When we engage in SETI we, and any civilisation out there trying to find us, are playing exactly this kind of game. So, if both we and they want to make contact, both of us can look to game theory to develop the best strategy," says Dr. Eamonn Kerins.

The new theory suggests examining transiting planets, planets that are on orbits that pass directly across the face of their host star, briefly making it appear dimmer. This dimming effect has been previously used to discover planets. In fact, transiting planets make up most of the planets we currently know about. For some, astronomers can determine if they are rocky planets like Earth, or if they have atmospheres that show evidence of water vapor.

"What if these planets are located in line with the plane of the Earth's orbit? They'll be able see Earth transit the Sun and they'll be able to access the same kind of information about us. Our planets will be mutually detectable." said Dr. Kerins.

The zone from which Earth is viewed transiting the Sun is known as the Earth Transit Zone. In his paper and it is estimated that there should be thousands of potentially habitable planets located in this zone.

Could game theory help discover intelligent alien life? (phys.org)
 
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