It’s quite a challenge to make an Earth-like world.
You need enough mass to hold an atmosphere and generate a good magnetic field, but not so much mass that you hang on to light elements such as hydrogen and helium.
You also need to be close enough to your star that you remain comfortably warm, but not so warm that all your water gets baked away. And then you need an abundance of short-lived radioisotopes (SLRs).
SLRs are isotopes with half-lives of less than 5 million years. That’s a blink of an eye at cosmic timescales, which means their decay helps warm up the early solar system.
The idea is that a warmer early solar system had enough heat to prevent terrestrial planets such as Earth from holding too much water.
Without SLRs most Earth-sized planets would become Hycean worlds. We know the solar system was rich in SLRs because of the isotopes we find in meteorites.
For example, the SLR aluminum-26 decays into magnesium-26. So if you find an excess of magnesium in a meteor fragment, you know radioactive aluminum was around long ago. The same is true for other radioisotopes such as titanium-44.
The only problem with this idea is that short-lived radioisotopes are formed in supernovae, and nearby supernovae would tend to rip apart the protoplanetary disk of a young star.
Somehow the Sun’s early disk survived intact, and if that is a rare thing, it would mean that Earth-like planets are rare. But a new study suggests they could actually be common.
The authors suggest that rather than being blasted by a nearby supernova shockwave, our early solar system was bathed in cosmic rays from a more distant supernova.
According to their model, if at least one supernova occurred within a parsec of us, it would bathe the solar system with enough cosmic rays to create the level of radioactive isotopes necessary to match those of meteorites.
Since sun-like stars form within star clusters, the odds of experiencing such a supernova are pretty good. This means terrestrial planets such as Earth should be fairly common.
We know that supernovae can enrich the galaxy with radioactive aluminum.
In fact, the level of aluminum-26 in our galaxy gives us a good estimation of the average rate of supernovae in the Milky Way. So this model is certainly plausible.
Written by Brian Koberlein/Universe Today.
