In our last episode, I talked about how researchers think the rise of oxygen levels here on Earth helped life come about and how we could use that rise as a potential biosignature on exoplanets. I also mentioned that there are possibly a lot of ingredients that went into allowing life to develop here, and we’re not sure which ones are necessary because of our sample size of one system with life.

And our Moon is unique in our own solar system. Its radius is over a quarter the size of Earth’s radius, which is a larger ratio than other moons and their planets. Our Moon stabilizes Earth’s spin axis, which could have helped create a great environment for the development and evolution of life. When looking for that as yet unfound Earth 2.0, perhaps we need to also find the Moon 2.0 to go with it.

To understand how necessary that aspect of the search may be, Miki Nakajima led a study of moon formations, which was published in Nature Communications, and concluded that only certain types of planets can form a moon similar in size ratio to the Earth-Moon system. Nakajima explains: By understanding moon formations, we have a better constraint on what to look for when searching for Earth-like planets. We expect that exomoons [moons orbiting planets outside our solar system] should be everywhere, but so far we haven’t confirmed any. Our constraints will be helpful for future observations.

Our Moon formed when a Mars-sized protoplanet struck the proto-Earth way back at the beginning of the solar system. This collision formed a partially vaporized disk of material around that proto-Earth, and that material eventually collected together and became the Moon as we know it. To understand how other systems might form in a similar fashion, the press release explains: Nakajima and her colleagues conducted impact simulations on the computer, with a number of hypothetical Earth-like rocky planets and icy planets of varying masses. They hoped to identify whether the simulated impacts would result in partially vaporized disks, like the disk that formed Earth’s moon.

And they discovered that rocky planets which are six times Earth’s mass and icy planets the mass of Earth will fully vaporize the disks in a collision, making the system incapable of forming a large enough moon. Nakajima concludes: We found that if the planet is too massive, these impacts produce completely vapor disks because impacts between massive planets are generally more energetic than those between small planets. As a result, we conclude that a completely vapor disk is not capable of forming fractionally large moons.

So a rocky Earth 2.0 needs to be smaller than six times the mass of Earth 1.0. That definitely narrows down the type of exoplanet we need to look for.

More Information

University of Rochester press release

“Large planets may not form fractionally large moons,” Miki Nakajima, Hidenori Genda, Erik Asphaug and Shigeru Ida, 2022 February 1, Nature Communications