In the hunt for a second Earth, look to small planets, says new research

Scientists around the world are constantly on the hunt for planets outside our solar system that could potentially provide a habitable environment for life.

Their telescopes, on the ground and in space, tend to focus on planets much larger than Earth, in part because they're easier to spot than smaller planets, and thus more conducive to study.

But new research from the University of Rochester published in The Planetary Science Journal suggests scientists ought to be on the lookout for planets that are not much larger than our own. A rocky planet with a large moon may have good potential to host life, given that our moon controls essential aspects of life, including the length of the day, ocean tides, and stable climate.

"Relatively small planets similar to the size of Earth are more difficult to observe and they have not been the major focus of the hunt for moons," says Miki Nakajima, an assistant professor of earth and environmental sciences at Rochester and the lead author of the research. "However, we predict these planets are actually better candidates to host moons."

The details of the origin of the Earth's moon are a longstanding debate in planetary science.

The prevailing theory is that the moon formed about 4.5 billion years ago when the Earth was hit by a planetary embryo the size of Mars. The collision threw into orbit a partially vaporized disk of debris that coalesced into the moon. Other models suggest that Earth collided with a much larger object, which would have produced a completely vaporized disk.

There are nearly 300 moons in our solar system, but their masses are typically much smaller relative to their host planets than our moon is to Earth—and that matters when it comes to the formation of life. Moons can form by other processes, but these moons are typically small compared to the sizes of their planets. In contrast, a huge impact tends to generate a massive moon.

While many scientists theorize that a moon isn't necessary for a planet to sustain life, they also acknowledge that our disproportionately large moon played a crucial role in the development of complex lifeforms on Earth.

After all, it is the moon's gravitational tug that is largely responsible for the tidal flow of the oceans, which scientists believe enabled the formation of the nucleic acids that fostered life as we know it. And the moon stabilizes Earth's orbital tilt, which keeps the climate relatively predictable so organisms can more easily evolve and adapt.

Scientists have detected more than 5,000 exoplanets—planets beyond our solar system. But exomoons—moons that orbit exoplanets—are elusive because they are, by nature, so much smaller than the planets they orbit. To date, only a couple plausible candidates have been identified.

That may matter in the search for a second Earth, one that could offer an ideal environment for life—and this is where the latest science conducted by Nakajima and her co-authors comes in.

Building on previous research that relies on computer simulations of moon formations, they investigated the role so-called "streaming instability" plays in creating moons.