— Earth's moon helped stabilize the planet's tilt, making its climate suitable for advanced life.
— Without a moon, life on Earth probably would have flourished as well, since Jupiter and other factors exert a steadying hand.
— The research has implications for understanding if planets in habitable zones are indeed suitable for life.
Scientists have long believed that without the moon's stabilizing gravitational influence, variations in Earth's tilt would have caused climate change too dynamic for complex life to evolve. Not so, concludes a new study that has implications for understanding conditions for life elsewhere in the solar system.
The study sprang from the ongoing Kepler Telescope mission to find Earth-like planets circling in habitable zones around other stars in the Milky Way.
"We were wondering 'Do we really have to find a moon or not?' around potentially habitable worlds, planetary scientist Jason Barnes, with the University of Idaho, told Discovery News.
Previous studies showed that without the steadying gravitational influence of a large moon, Earth's tilt would shift by as much as about 85 degrees every 100,000 years or so, alternatively freezing and baking the planet's poles. Scientists believe a stable climate spanning about 500,000 years was necessary for complex life to blossom on Earth.
A new computational analysis, however, shows that a moonless Earth would still have swings in its tilt but the influence of Jupiter and other factors would limit the variations to about 10 degrees in either direction.
Earth's rotational tilt varies between 0.5 and 1 degree about every 100,000 years.
"Plus- or minus-10 would certainly be noticeable and may be a problem, but I don't think it would prevent life from coming about," Barnes said.
"It's a very intriguing result. It's provocative," Richard Vondrak, lead scientist with NASA's ongoing Lunar Reconnaissance Orbiter mission, told Discovery News.
"On the moon we can find important evidence and clues of what happened to not only the moon, but also to the Earth-moon system over the last 4.5 billion years," said Vondrak, a planetary scientist with NASA's Goddard Space Flight Center in Greenbelt, Maryland.
The study also showed that if Earth revolved around the sun in the opposite direction, called a retrograde orbit, it wouldn't need a moon at all to have a climate about as stable as it has today. Likewise, a Jupiter about half the distance to Earth as its present location would have had a similar steading hand, Barnes added.
The findings are causing extrasolar planet hunters to revise their thinking on what constitutes a habitable planet.
"We think that at least 80 or 90 percent of planets out there statistically won't even require a moon" to have a stable climate, Barnes said.
Location is key. In our own solar system, Mars shows evidence of extreme climate change, the result, scientists believe, of a rotational tilt that flips between zero and 60 degrees over time. A big moon likely could have helped stabilize Mars' orbit, but the planet has just two small moons, most likely captured asteroids, that don't have much gravitational muscle.
Other factors impacting a planet's climate and suitability for life include its star's age, composition and the size and location of sibling planets in the system.
"It's a very complex problem for sure and we're not anywhere near solving it, but we're making positive steps as we slowly evaluate each of these conditions and discover what constraints they really place on whether life can exist there," Barnes said.
The research appears in this month's Astrobiology magazine and is pending publication in the journal Icarus.