Having an unstable tilt was thought to create inhospitable conditions on a planet. After all, Earth has a fairly stable tilt of 23.5 degrees, as well as the stabilizing influence of a large moon. A new study challenges that long-held view, showing how a fluctuating tilt in a planet’s orbit may actually help life along.
These “tilt-a-worlds,” which turn from their orbital plane due to the influence of companion planets, are less likely than fixed-spin planets to freeze over. That’s because heat from their host star is more evenly distributed. Specifically, this only happens at the outer edge of a star’s habitable zone -- where rocky exoplanets can maintain liquid water at the surface. Further out, a “snowball state” of global ice is inevitable.
Earth and our neighbor planets occupy roughly the same plane in space. In systems where planets ride along at angles to each other, they tend to tug on each other from above or below. This changes their poles’ direction relative to their central star.
To see what an Earthlike planet might do if it had similar neighbors, a team led by John Armstrong of Weber State University used computer simulations to reproduce off-kilter planetary alignments.
With extreme variations in obliquity (or the angle of the planet’s spin axis), the wobble causes the poles to occasionally point toward the host star -- causing ice caps to melt quickly. “Without this sort of ‘home base’ for ice, global glaciation is more difficult,” study coauthor Rory Barnes of University of Washington says in a news release. “So the rapid tilting of an exoplanet actually increases the likelihood that there might be liquid water on a planet’s surface.”
Minus the moon, Earth’s tilt might increase by 10 degrees or so. While climates might fluctuate, liquid water and the life it sustains would still be possible. “We’re finding that planets don’t have to have a stable tilt to be habitable,” Barnes says. In fact, “the presence of a large moon might inhibit life, at least at the edge of the habitable zone.”
The new findings dramatically expand the outer edges of the habitable zone, perhaps doubling the number of potentially habitable worlds in the galaxy. In the context of our own neighborhood, “it would give the ability to put Earth, say, past the orbit of Mars and still be habitable at least some of the time,” Barnes adds, “and that’s a lot of real estate.”
The work was published in Astrobiology this week.
[Via University of Washington]
Image: NASA/Goddard Space Flight Center