As you mark the longest day of the year, consider the debate among astronomers over whether Earth’s tilt toward the sun helps make life on our world and others possible.
By Shannon HallJune 18, 2021
This article, originally published in 2018, has been updated to reflect 2021’s solstice date.
On the summer solstice this Sunday, the Northern Hemisphere will dip toward the sun and bathe in direct sunlight for longer than any other day of the year. That will cause the sun to rise early, climb high into the sky — sweeping far above city skylines or mountain peaks — and set late into the evening.SIGN UP FOR SCIENCE TIMES: Get stories that capture the wonders of nature, the cosmos and the human body.Sign Up
The solstice occurs because Earth does not spin upright but leans 23.5 degrees on a tilted axis. Such a slouch, or obliquity, has long caused astronomers to wonder whether Earth’s tilt — which you could argue is in a sweet spot between more extreme obliquities — helped create the conditions necessary for life.
It’s a question that has been brought to the forefront of research as scientists have discovered thousands of exoplanets circling other stars within our galaxy, bringing them closer to finding an elusive Earth 2.0. Is life only possible on an exoplanet with a tilt similar to ours? Or will life arise on worlds that spin straight up and down like spinning tops or on their sides like a rotisserie chicken? And what if a world swings between two axial tilts? The answer is far from simple.
Although astronomers have yet to detect an exoplanet’s tilt, they suspect that they will vary wildly — much like the planets within our own solar system. Mercury at 0.03 degrees hardly slouches, while Uranus leans on its side at 82.23 degrees. Those are two extremes that are far from habitable, even if those worlds looked like Earth in all other regards, said René Heller, an astronomer at the Max Planck Institute for Solar System Research in Germany.
If the planet had no slouch, it wouldn’t have seasons. The hemispheres would never dip toward or away from their star. Instead, the poles (which always point toward the frigid depths of space) would be so cold that carbon dioxide would be pulled from the sky, an effect, Dr. Heller argues, that would cause the planet to lose its precious greenhouse gas so that liquid water could never form.
But if the planet spun on its side, life might also be hard to come by. There, the poles alternatively point directly toward and away from the host star, causing one hemisphere to bathe under the sun both day and night during that long summer, while the other hemisphere experiences a frigid and dark winter — before the seasons flip. Although such a planet might not necessarily lose its liquid surface water, any life would have to adapt to a world that permanently switches between boiling and freezing.
Dr. Heller argues that the optimal tilt runs from 10 to 40 degrees. As such, there are several knobs that must be tuned to allow life and, Earth’s mild obliquity is one.
Rory Barnes, an astronomer at the University of Washington, disagrees. “There’s nothing special about 23.5 degrees,” he said. “You could have any obliquity and you could still have habitable conditions on the surface of the planet.”