Physics, Earth & Space Icon Physics, Earth & Space

Exoplanets and the Fermi Paradox

Guillermo Gonzalez

Fermi Paradox

We are living during a golden age of discovery in astronomy. Arguably, it began with the dawning of the space age in 1957. By 1989 our probes had visited every planet in the Solar System (in 2015 New Horizons visited the former planet Pluto). Then, in 1995 we discovered the first planet around another star (an exoplanet). 

The Extrasolar Planets Encyclopedia listed 3,986 exoplanets as of February 15, 2019! The Kepler spacecraft has discovered the most, but it was retired last fall after running out of the fuel needed to do science observations. Its timely replacement, the Transiting Exoplanet Survey Satellite (TESS), began its science observations last summer. It is expected to discover over 20,000 planets. The ongoing Gaia mission is expected to yield about 20,000 giant planet discoveries by the time its nominal 5-year mission is completed later this year.

Believers in extraterrestrial intelligence (ETI) have been encouraged by these discoveries (see here and here). But the number of stars with planets is only one of the seven factors in the Drake Equation, which is an attempt to estimate the number of communicating civilizations in the galaxy. One also needs to consider the many Rare Earth factors needed to make a planet habitable to complex life (see here and here). It could be that these factors more than compensate for the large numbers of planets, resulting in a very small chance of ETI. We just don’t know yet.

The Great Silence

Do these discoveries help resolve the Fermi Paradox, which asks, Why the Great Silence? Not really, but they do show that mere rarity of planets by itself is not the solution.

There is one important, albeit indirect, way that exoplanet discoveries do influence the Fermi Paradox. To understand it, we need to think “backwards.” Consider not our detection of exoplanets but, rather, the detection of our Solar System from afar. Now that we know how to find exoplanets, we can turn the problem around and ask how easy would it be for an ETI to detect the planets in our Solar System.

Right away we can offer some obvious answers. For example, you don’t actually have to send a probe to another star to check if it has planets. The technology required for long-range detection has been available for decades. There are multiple ways to detect exoplanets, but the two most productive primary ones are the spectroscopic Doppler and photometric transit methods. In 1995 the first exoplanet around an ordinary star was discovered, using the Doppler method. Planets could have been discovered using this method one or two decades prior. In principle, exoplanets could have been discovered a century ago using the simpler transit method. 

Notable Properties of the Sun

The Sun possesses a couple of properties that make it an excellent target for planet searches from afar. First, it is an old star and, as such, exhibits slow rotation and small fluctuations in brightness. These qualities are beneficial for both primary planet detection methods. Jupiter would be the easiest Solar System planet to detect using the Doppler method, while Venus would be the easiest one to detect with the transit method, given its relatively short orbital period. One qualification regarding the transit method is that the observer would have to be looking near the ecliptic plane of the Solar System. We have already found several Jupiter and Venus analogs, proving that we could detect at least a few planets around a nearby Solar System twin.

Second, the Sun is a relatively rare early-G spectral type main sequence star, making it brighter than about 90 percent of all main sequence stars. ETI conducting a magnitude-limited survey could see the Sun from farther away than they could an intrinsically fainter but more common red dwarf. Or, for a given distance, better quality data could be gathered on the Sun, making it more likely that its planets would be detected.

A Rare Jewel Sure to Enthrall!

All this is interesting from the perspective of a space-faring ETI searching for other habitable or inhabited worlds. An advanced ETI just starting to go boldly out into interstellar space would very likely have mapped out all the planetary systems within the nearest few hundred light years of their home system. It seems likely that our Solar System would be included in their catalog of planetary systems (assuming they start out in our sector of the Milky Way galaxy). 

If all planetary systems were created equal, then the Solar System would not stand out. ETIs might eventually visit it, but it wouldn’t be a high priority. But all planetary systems are not created equal. The many Rare Earth factors show that the Solar System is anything but typical and that only a small subset of planetary systems are likely to be habitable (and far fewer actually inhabited). 

Just knowing that our Sun is a rare early-G spectral type main sequence single star orbiting far from the dangerous galactic center would substantially increase the Solar System’s ranking in the ETI’s travel itinerary. Knowing further that the orbits of the planets in the Solar System are nearly circular would boost its rank more. Of course, a space-faring ETI would probably know more about exoplanets than we do, given their more advanced technology.

Still, the technology we use to study exoplanets is advancing rapidly. For example, the James Webb Space Telescope, due to be launched in 2021, should be able to detect whether nearby exoplanets have oceans and ozone. Knowing that the Earth has oceans of water, an oxygen-rich, carbon dioxide-poor atmosphere, and a large moon should boost the Solar System’s rank to the top of the ETI’s list. Their travel brochure might describe it as a rare jewel that is sure to enthrall!

Not Likely to Be “Passed Over”

All this strongly implies that the Earth would not have been “passed over” during a “colonization wave” through the galaxy. Yet, there is no convincing evidence of ETI visitation or communication. Yes, I know there are speculative responses to the Great Silence in attempts to rescue ETIs from the obvious implications of the Fermi Paradox. There are good responses to these. I would recommend If the Universe is Teeming with Aliens … Where is Everybody? (2nd edition). Contrary to first impressions, then, exoplanet discoveries actually strengthen the impact of the Fermi Paradox.

Image: An exoplanet (artist’s rendering), by ESA/Hubble [CC BY 4.0], via Wikimedia Commons.