The Earth is not only habitable, it is exceptionally habitable. It’s rather humid for a planet so close to its Sun, it’s geologically active and it has a stable orbit, all necessary for life as we know it. But there are also secondary advantages, such as not being constantly bombarded by large asteroids, and having a fairly stable axis of rotation. This is due in part thanks to the planet Jupiter. The giant planet helped clear the solar system of asteroid debris and may have helped stabilize the orbits of the inner planets. So life is beautiful. But a new study shows that if Jupiter had a different orbit, life could be even better.
The study looked at how Jupiter affects Earth’s orbit and axial tilt over time. Both of these factors are important to our weather and our climate. For example, the Earth has a very circular orbit. In mathematical terms, the shape of an orbit is measured by its eccentricity. At e=0, the orbit is a perfect circle, and the more elliptical an orbit is, the closer e approaches 1. Earth’s orbit has an e=0.017. This means that the Earth is slightly closer to the Sun for just under half of the year, and slightly further away for just over half. This is because the Earth moves slightly faster in its orbit when it is closer to the Sun rather than farther away.
At this point, astute readers will be eager to point out that this quirkiness isn’t why we have seasons, and they’d usually be right. Our regular seasons are caused by the Earth’s axial tilt, which is currently about 23.4 degrees. Because of this tilt, the Sun is higher in the summer and lower in the sky in winter. And, of course, when it’s summer in the northern hemisphere, it’s winter in the southern hemisphere.
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Interestingly, for the northern hemisphere, summer is when the Earth is a bit farther from the Sun, meaning northern summers are slightly longer than southern summers. Since more of Earth’s landmass is in the Northern Hemisphere, this means Earth is a bit more habitable than it would be if we had a circular orbit. It does not affect our daily lives, but it is played out on a geological scale.
Over time, the Earth’s axial tilt changes slightly, resulting in more extreme or mild winters. Earth’s axis also precedes over time, meaning its orientation relative to the elliptical shape of Earth’s orbit also shifts. All of these factors underlie what is known as the Milankovitch cycle. It is a measure of the overall amount of heat received by the Earth’s land surface, known as insolation. It varies over thousands of years. Twelve thousand years ago, at the dawn of human civilization, the insulation was very high, offering us a particularly pleasant world. Currently, it is a little lower, and without the effects of global warming, we would be in a cool period.
Part of the Milankovitch cycle is due to Jupiter’s slight gravitational pull. But since Jupiter also has a circular orbit (e = 0.048), this is not a significant factor. In this recent study, the team created simulated solar systems where Jupiter’s orbit had a higher eccentricity. They thought a more eccentric Jupiter would make Earth less habitable, but were surprised to find that it actually made things better. With increased gravitational effect from Jupiter, Earth would have better insolation on its surface, so that even more of Earth’s landmasses would be in a temperate range.
This has big implications for potentially habitable worlds in other star systems. Although we tend to focus on whether a world is in the habitable zone, that’s only the first requirement for a truly habitable world. Other factors such as insolation depend on the presence of other planets in the system. There is a gravitational dance between worlds that can make or break a planet’s life chances.
Reference: Vervoort, Pam et al. “System architecture and planetary obliquity: implications for long-term habitability.” The Astronomical Journal 164.4 (2022): 130.
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