This article, originally published in 2018, has been updated to reflect 2020’s solstice date.

On the summer solstice this Saturday, 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.

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.

[Get a reminder on your calendar for the solstice and other astronomical events.]

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.