The discovery of life on other planets is the holy grail for astronomy. It would resolve one of the biggest paradoxes in science: that, as the universe expands and takes in more and more matter, there is always a finite amount of solar system-sized planets around other stars. How could there be just one or maybe two such planets around every star? For more than a century the prevailing hypothesis for why this limit exists has been that planets with larger orbits and higher pressures and temperatures are more likely to develop life.
These larger planets also tend to be older or closer to their stars and have higher surface gravities, which could make for more vigorous radiation and the development of complex chemical processes. In the first half of the 20th century, however, a different theory emerged that challenged not only this existing theory, but also several theoretical models of how planets might be made.
On June 17, 1974, physicist Paul Davies won the Nobel prize for showing that if you took an idealized planetary system that had planets of perfect size and shaped, with temperatures so near the freezing point that only ice molecules would exist in the liquid metallic state, the planet would not last long. The planets would form into hot little rocky planets just 10 to 20 kilometers across — roughly the same size as Mercury, Venus, or Mars.
As the planets heated and moved away from their parent star, they would lose atmosphere, which is why the temperature would gradually drop as those planets moved further away. (The planets must be sufficiently hot to hold onto their atmosphere at such distances, otherwise the star would cool and the planet would vaporize.) According to one of the favored scenarios, the planets that formed in this system would have been at least 2.5 billion years old.
In 1998, however, astronomers discovered more planets in Kepler-13, which orbited a sun-like star in the constellation Cygnus. These found planets larger both in area and mass than those they had thought there could be around stars like the sun. They have been dubbed super-Earths. The two largest, which orbit their sun in only 8 and 2.1 days, have been dubbed Kepler-22b and Kepler-22c. They are more than four times bigger than Earth. There is little doubt that life could indeed exist on planets like these and it is also possible that it does not.
Still, these discoveries suggest that Earth, far from being abandoned by its inhabitants, could be a very successful planet indeed. If we could be sure that these planets are rocky like ours then at least that would go some way to convincing sceptics that we haven’t been in some sort of cosmic purgatory and indeed perhaps give us some new reason for hope that life does not have to be dead.
But at some point in the future, when the James Webb Space Telescope, a successor to the Hubble, is launched (Set for: 31 October 2021) and we can look into the distant skies again, I believe that we should start looking into even more distant stars – even ones which astronomers could never see directly, but which we can infer exist by what we know of the planet’s gravitational field from our own system.
With some luck, we might actually be able to detect some kind of signal from intelligent life, even if we don’t yet know that we should be looking for it. It’s possible our best chances of getting some evidence of alien life may just come by looking.
As of 2020 Astronomers have still not detected any signs of life, however, The search will not just stop and the discoveries of Earth-like planets can be a powerful motivation to keep us looking, because it means that somewhere in the universe, out there in the vast amount of space is another world that could potentially offer alien life.
The question is what form of life do we want to find, or could it be too different for us to grasp, a challenge to our capacity to understand? For all the debates about what life actually is we still are not certain about what the best label for it is. And even if we could find evidence of life on another planet, how would we even be able to reach it when we still haven’t been able to put a man on mars.