Does life exist beyond Earth? Not surprisingly, searches of the solar system yield no evidence because the only planet (or moon) located in the right place is Earth. However, the discovery of exoplanets (planets outside our solar system) boosted researchers' enthusiasm for finding life beyond Earth. Thus far, our technology lacks the sensitivity to detect any signatures of life outside of our solar system, but scientists continue to make progress. While these advances reveal additional indicators that Earth may be rare (or unique) in its capacity to support life, they also provide a way to genuinely test the rare-earth hypothesis.
Using current telescope technology, scientists can measure only orbits, masses, and sizes of exoplanets. Over the next decade, advances will permit the detection of life signatures from stars in the neighborhood of the solar system, which will allow powerful tests of the rare-earth hypothesis. An article published in the journal Astrobiology highlights one of those tests.
Earth currently resides near the habitable zone’s inner edge. Yet, as the Sun’s luminosity increases (with age), the habitable zone will move farther and farther out, making Earth less habitable. As Earth’s surface temperature increases, the planet will go from supporting large-bodied, complex life (like humanity) to hosting only multicellular life and then only microbial life. Eventually it will be completely devoid of any life. The whole progression takes between 2 to 3 billion years. Although Earth provides an impractical venue for testing this idea, older exoplanets in the solar neighborhood serve as good places for the test.
As described in Astrobiology, researchers identified six Sun-like stars that could host Earth-like planets residing in the continuous habitable zone (CHZ). Each of these stars measures between 6 and 7 billion years old (as compared to the 4.6-billion-year age for the Sun and Earth). Applying biosphere evolution models to (hypothetical) planets orbiting the stars revealed that each planet was in a different stage of decline from Earth-like to almost extinct. Future measurements to find planets around these stars and characterize their biosignatures will yield constraints on the validity of the rare-earth hypothesis.1
Even though better testing is still a ways off, the current research does highlight one more criterion that habitable planets must meet. Not only must they start in the habitable zone, they must remain within a habitable zone that continually moves farther away from the host star. As the Astrobiology article states, “If the development of Earth-like biospheres is rare, requiring a sequence of low-probability events to occur, biosphere evolution models suggest they are rarer still, with only thousands being present in the Galaxy as a whole.” Research continues to point to a rare, finely designed Earth.