Moons similar to Earth’s seem to be vital for life on other planets. Find out why a new study explains how to narrow our search for worlds with suitable natural satellites.
I remember the first time I watched a lunar eclipse. It was about 35 years ago, and I was living in Ottawa at the time.
I walked over to Dundonald Park, where I thought I would have a better view. I discovered I wasn’t the only one looking forward to the display.
The park was filled with an array of characters from young families to bohemian hipsters. Bongos and acoustic guitars rang out as we waited for the Earth to pass between the sun and the moon.
During Eclipse, Moon Darkened and Turned Blood Red
The crowd went silent as the Earth’s disk began to obscure the moonlight. The Moon darkened and turned blood red once it could no longer reflect any sunlight.
We seem to feel an innate attraction to the Moon and its phases. One reason for that may be that we sense that its relationship with the Earth is essential to life.
Science bears out this intuition. Compared to other planets in the Solar System, our Moon is unusually large compared to the size of our planet. Its radius is more than one-quarter of the Earth’s radius, which is the largest ratio within our Solar System.
Tides Could Have Played Role in Development of Early Cells
Scientists speculate that the powerful tides the moon creates could have played a role in the development of the earliest living cells. The moon also stabilizes the way our planet tilts on its axis, which regulates our climate.
Earth’s tilt only varies by about one degree. However, the tilt on Mars, for example, seems to have varied between 0˚ and 60˚ over the past few million years.
It’s not just a matter of the moon being relatively large. It it were any larger it would have the opposite effect, destabilizing Earth’s axial tilt. Its mass seems to be “just right” to keep our planet in line.
Wouldn’t Other Planets Need a Satellite Similar to Ours?
The role of the Moon in supporting our natural world has sparked a debate among astrobiologists. If the Moon is so essential to life on our planet, wouldn’t other planets need a satellite similar to ours?
If so, how common are Moons like the one we see passing through its phases at night? Does our need for our right-sized moon make life extremely rare in the Universe?
A new study published in Nature Communications this week sheds some light on this controversy. It concludes that only certain kinds of planets have what it takes to form a moon similar to ours.
Only Certain Planets Can Form a Moon Like Ours
Miki Nakajima is an assistant professor of earth and environmental sciences at the University of Rochester. She led the study and she explained its goal this way.
“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 happened to form by coincidence. An object named Theia, about the size of Mars, collided with Earth and caused a partially vaporized disk to form around the Earth. That disk gradually solidifed into the Moon.
Computer Simulations of Planetary Collisions
The researchers created computer simulations of collisions between hypothetical Earth-like planets and various sized icy objects like Theia. They wanted to see if the computer models produced the same kind of partially vaporized disks around their simulated rocky planets.
They found that if an icy planet more massive than the Earth collides with a rocky planet more than six times the mass of the Earth, the resulting disk is fully vaporized. To form into a moon, the disk should only be partially vaporized, so this seems like a upper size constraint on lunar formation.
“We conclude that a completely vapor disk is not capable of forming fractionally large moons,” explained Professor Nakajima. “Planetary masses need to be smaller than those thresholds we identified in order to produce such moons.”
We’ve All Wondered if We’re Alone in the Universe
We’ve all gazed at the stars in the night sky and wondered if we’re alone in the Universe. It’s been a fundamental human question for as long as we’ve know there were other worlds out there.
Our traditional stories about how life began and our place in the world have lost their appeal as science has progressed. We all need a new story we can tell each other about the world around us and our place in it.
Since scientists discovered the first exoplanets back in the early 90s, they’ve catalogued more than 4,000 worlds orbiting other stars. The challenge for those trying to fill in the blanks in our story line is to narrow these exoplanets down to those that could support an ecosystem.
Narrow Exoplanets Down to Those that Could Support Life
So far, no astronomer has observed a moon orbiting a planet outside our solar system. This new study suggests that this could be because we’ve been looking in the wrong place, like the person looking for their keys under the lamp post because it’s easier to see there.
The conventional wisdom has been to seek out very large planets to find the ones with moons. This new study suggests there’s a better approach.
Professor Nakajima wrapped up the discussion by suggesting, “The exoplanet search has typically been focused on planets larger than six earth masses. We are proposing that instead we should look at smaller planets because they are probably better candidates to host fractionally large moons.”
We always have more to learn if we dare to know.
Learn more:
Moons may yield clues to what makes planets habitable
Large planets may not form fractionally large moons
Astrobiology: 3 Questions We Need to Answer
Earth-Like Planets Orbit Half of All Sun-Like Stars
Intelligent Life on Other Planets: Odds of Finding It
