how far away could we detect that Earth has life?

Question

My guess is that life bearing planets are too far apart to be detected. I think we can only find the ones within a sphere around our planet that is 100s of light years in diameter but I suspect life bearing planets may be much farther away than that.

I would like to estimate the diameter of the sphere within which we could detect life on another planet and then estimate the probability that there is life within that sphere.

For example, give our current technology what is the furthest distance would be able to detect life on Earth? How many stars like our sun are in that sphere? How long would it take for SETI to rule out each of those stars?

Answer 1

I had put off answering this question because it seems too broad without specifying what sort of detection methods are proposed. But if you answer it directly from the perspective of - if we were to take the solar system and put it at some distance from us, would we able to detect signs of life on planet Earth - then the answer is probably not.

Using current technology (and by that I mean experiments and telescopes that are available now) we would probably be unable to detect life on Earth even if observed from a distance of a few light years. Therefore there are no stars within this sphere (other than the Sun).

1. No planets quite like the Earth have yet been detected around another star. That is to say, none that have a similar mass, radius and orbit at 1 au (or close to it) from a solar-type star [EDIT: There is of course now a close contender in Kepler-452b, though it is 60% bigger than Earth; Jenkins et al. 2015.]. With current technology, it is just about in reach. Therefore any directed search for life on Earth has a limited number of places in which to start. If you can't detect the planet at all then there is absolutely no chance of looking at its atmospheric composition to look for biomarkers (e.g. oxygen along with a reducing gas like methane, or chlorofluorocarbons from an industrial civilisation - Lin et al. 2014). The only exoplanets for which atmospheric compositions have been (crudely and tentatively) measured are "hot Jupiters". - giant exoplanets orbiting very close to their parent stars.

2. A "blind" search could look for radio signatures and of course this is what SETI has been doing. If we are talking about detecting "Earth", then we must assume that we are not talking about deliberate beamed attempts at communication, and so must rely on detecting random radio "chatter" and accidental signals generated by our civilisation. The SETI Phoenix project was the most advanced search for radio signals from other intelligent life. Quoting from Cullers et al. (2000): "Typical signals, as opposed to our strongest signals, fall below the detection threshold of most surveys, even if the signal were to originate from the nearest star". Quoting from Tarter (2001): "At current levels of sensitivity, targeted microwave searches could detect the equivalent power of strong TV transmitters at a distance of 1 light year (within which there are no other stars)...". The equivocation in these statements is due to the fact that we do emit stronger beamed signals in certain well-defined directions, for example to conduct metrology in the solar system using radar. Such signals have been calculated to be observable over a thousand light years or more. But these signals are brief, beamed into an extremely narrow angle and unlikely to be repeated. You would have to be very lucky to be observing in the right direction at the right time if you were performing targeted searches.

Hence my assertion that with current methods and telescopes there is not much chance of success. But of course technology advances and in the next 10-20 years there may be better opportunities.

The first step in a directed search would be to find planets like Earth. The first major opportunity will be with the TESS spacecraft, launching in 2017, capable of detecting earth-sized planets around the brightest 500,000 stars. However, it's 2-year mission would limit the ability to detect an Earth-analogue. The best bet for finding other Earths will come later (2024 perhaps) with the launch of Plato, a six-year mission that again, studies the brightest stars. However, there is then a big leap forward required to perform studies of the atmospheres of these planets. Direct imaging and spectroscopy would probably require space-borne nulling interferometers; indirect observations of phase-effects and transmission spectroscopy through an exoplanet atmosphere does not require great angular resolution, just massive precision and collecting area. Spectroscopy of something the size of Earth around a normal star will probably require a bigger successor to the James Webb Space Telescope (JWST - launch 2018), or even more collecting area than will be provided by the E-ELT in the next decade. For example Snellen (2013) argues it would take 80-400 transits-worth of exposure time (i.e. 80-400 years!) to detect the biomarker signal of an Earth-analogue with the E-ELT!

It has been suggested that new radio telescope projects and technology like the Square Kilometre Array may be capable of serendipitously detecting radio "chatter" out to distances of 50 pc ($\sim 150$ light years) - see Loeb & Zaldarriaga (2007). This array, due to begin full operation some time after 2025 could also monitor a multitude of directions at once for beamed signals. A good overview of what might be possible in the near future is given by Tarter et al. (2009).

Answer 2

Depends on what you mean by detecting life. As is explained in this what-if post by Randall Munroe, the algae on earth will tell the aliens about us before we can tell them about us.

If you consider the presence of liquid water or the presence of $O_2$ as detecting life, then such detection can be made by studying the spectrum of extra-solar planets, measurements that we can currently make. The furthest extra-solar planet discovered so far is at a distance of 27,700 light years. So, a partial answer to your questions would be to study the spectrum of every extra-solar planet found within the circumstellar habitable zone to look for signatures of tell-tale signs of life. We currently do have the technology to measure the optical reflection spectrum of an extra-solar planet, for example ESO's VLT, the Gemini Observatory and the OSIRIS instrument on GTC but I don't know if SETI has that capability. You could further look up the work of Dr. Sara Seager.

Answer 3

I'm finding this very tough to answer, the method of detection is critical in how far we can detect. There are two probable methods I can think of, one superior to the other. The first method involves the speed of light and our production of waves. The second involves how we have tailored our atmosphere.

Our production of waves (radio) started late 19th century, if we use a point of reference, say 1900; we have been broadcasting for 115 years, at light speed a species no further than 115 light years away could detect us. Hence the idea of the SETI program as Rahul has suggested, with the intention of broadcasting ourselves.

The best method, and the one I can see working for humans in their search for others, is atmospheric poisoning. There are specific hydrocarbons in our atmosphere which are thought to only be produced by man, if we think like this, then it's plausible we may also detect atmospheric poisoning around an exoplanet. Detecting oxygen simply is not enough, as it's not indicative that life exists, oxygen can be produced naturally in limited quantities as found elsewhere in the solar system however to sustain carbon based lifeforms such as ourselves there would have to be a major abundance. Detecting pollutants is the more logical way to conceive detection. If we are able to produce elements not naturally found, it is a clear indication that a species put it there. This also relies on the speed of light, however man made pollutants have existed pre-wave era, and have had longer to transmit light than our production of waves. The downside is detection method of the pollutants, currently as humans we rely on either using a star with a transiting planet to determine composition, or less precisely spectrum data (which doesn't indicate atmospheric material).

Another viewpoint is looking at the Kardashev scale, one could put forth that we have the technology to determine that answer based on energy consumption. If we could detect a massive gravitational field and no apparent source of energy, the energy may well be harvested by another species; such as a Dyson sphere. Such a detection I believe would be too easy to overlook as it's not something our species is actively searching for. While this holds truer for more of a theoretical detection, another species may be able to detect energy consumption on our planet, through lighting of our planet and atmosphere along with increasing surface temperatures.

I believe at best, as for human interference, we may be looking in the range of 100-150 light years. As for detection of life in general, I can't imagine pre-modern era if there was a simple way to determine that life existed if viewed from elsewhere apart from the fact we had a stable system containing liquid water and atmospheric oxygen.

We may be too reliant on providing the argument from our point of view being carbon based lifeforms, if another species advanced or more than us wasn't carbon based, it could very well be they are looking for other indications more localized to their own species, in the same way we look for indications which we imagine detecting ourselves with.

EDIT: As requested by Rob Jeffries; NO, using transit photometry using today's current technology is not yet possible. At 1ly Earth would appear as 2.776*10^-4″->3600*(180/π)*(12734/9.460*10^12) or 2.776mas, which is possible by ESO's Very Large Telescope which has an angular resolution able to image in milliarcseconds. At 10ly Earth would appear as 2.776*10^-5″->3600*(180/π)*(12734/9.460*10^13) or 277.6μas, possible after completion of the Cherenkov Telescope Array which has an angular resolution able to image in microarcseconds. Whilst the Cherenkov Telescope Array, is limited to 100μas at 400nm and not able to image 1μas, at this next level it's we are imaging at 100ly. The Gaia spacecraft can resolve up to 20μas however is not able to image at this level. The NASA Ames Research Center is demonstrating resolution abilities down to 5μas at the attempt to resolve down to 1μas, however again that is not imaging resolution. For radio waves, truly enough I had not mentioned the inverse square law and wave degradation. For us as humans, yes a few light-years may be possible with a realm of possibility opening up with the Square Kilometer Array.

If you want me to retract my guesstimate from the first time, pollution and transit photometry are in fact possible using today's existing technology within 1ly, on par with existing radio receivers within 1yr. If you deter from the fact that the new instruments are however not built yet you can vastly increase this up to 100ly, just because something isn't built does not make the technology non-existent (Is the SKA technology feasible? Yes, we have the technology to make it happen right now, we just haven't done so. That does not make it technology that does not exist).

Seti Home has published finding of the the first earth sized planet detected from transiting. Further publication by Cornell University Library claims that the planet is within the habitable zone and implies it's within possibility of having an atmosphere and liquid H20 on it's surface. The Kepler spacecraft detected this finding, in case you are unaware, Kepler maps light curves as a body transits across the face of another body, this is called Transit. To even suggest that this technology doesn't already exist is preposterous, if you want a true analogue to Earth as is, with technology already existing; 1ly, if you want to use technology possible however not built; 100ly.