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This has been kicking around in my head for a while. We've been detecting planets for decades by observing regular dips in starlight from many light years away as a planet transits its host star. I've often wondered if we considered staring at our own planet the same way to see if we can get our "Eureka! We found life!" moment.

After reading Carl Sagan detected life on Earth 30 years ago—here's how his experiment is helping us search for alien species today, I started searching online for more experiments aimed at detecting life on Earth. I came across Hubble Makes the First Observation of a Total Lunar Eclipse By a Space Telescope where we were able to detect ozone in Earth's atmosphere.

So, the Galileo spacecraft detected life as it flew by Earth thousands of miles away. Hubble stared at the moon during an eclipse, but that's a bit like knocking on the door to see if someone is alive, cosmologically speaking. Detecting those same signatures from trillions of miles away is such a shockingly different order of magnitude that it renders those experiments fun, and informative, but not small scale replicas of how we typically find exoplanets.

Are there plans to detect life on Earth the same way we think we can detect life on exoplanets, but on a much smaller scale? I'm imagining something at Neptune-ish distances as a test for how well these biosignatures can be detected at longer distances for an Earth-sized planet.

(Is this even a worthwhile experiment, given the logistics mentioned in Space telescope located in outer solar system?


I also realize I'm asking to build a telescope and send it 2.8 billion miles away just to snap a selfie, but we've already spent quite a bit of money building observatories to detect transiting exoplanets. It would be curious to know if these signatures are even detectable from light hours away, much less many light years away.

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    $\begingroup$ It would be very difficult to justify the expense, and 4 or 5 light-hours away is practically next door compared to exoplanet distances, so it wouldn't really test our exoplanet detection skills. But if we did put a 'scope in the outer system (eg for improved parallax measurements), then sure, we might like to try taking a selfie with it. OTOH, that could be tricky, since at 30 AU, the Earth is <2° from the Sun. $\endgroup$
    – PM 2Ring
    Oct 24, 2023 at 7:57
  • $\begingroup$ @PM2Ring on the contrary, one can observe the earth/Sun system in almost exactly the same way. But you need to be far enough away that you can observe transits and do transmission spectroscopy. Similar science has been done on from Earth, observing Venus. $\endgroup$
    – ProfRob
    Oct 24, 2023 at 17:05
  • $\begingroup$ @ProfRob Fair point. I'm just concerned that the Sun is still rather bright at 30 AU, so you'd need to be careful. $\endgroup$
    – PM 2Ring
    Oct 24, 2023 at 17:11
  • $\begingroup$ A basic issue here is that we don't detect life on exoplanets. We just detect exoplanets and some properties which may or may not indicate life is possible according to some models. $\endgroup$ Oct 29, 2023 at 12:21

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Somebody can fill in the details perhaps but both the Galileo and Osiris-Rex spacecraft analysed light received from the Earth and looked for the spectroscopic signatures of carbon dioxide, oxygen, methane and ozone. See for example here. However, these were data obtained from relatively close to the Earth.

Plans to observe the Earth in almost exactly the way we might be able to analyse exoplanets around other stars are being discussed. For example, Mayorga et al. (2021) argue that it is essential that we send a spacecraft to beyond the Earth-Sun L2 point so that it can look back and examine the Earth's atmosphere using transit spectroscopy in exactly the same way that JWST is doing for exoplanets.

A pointer to the kind of science we are talking about comes from the observation of Venus as a transiting planet that were done (from Earth) in 2004 and 2012 (e.g., Hedelt et al. 2011; Ehrenreich et al. 2012; Chavassa et al. 2015), though there has not been as much work on this as I imagined.

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