Earth is not only habitable, it is unusually habitable. It’s quite humid for a planet so close to its sun, it’s geologically active, and it has a stable orbit, all of which are necessary for life as we know it. But there are also secondary benefits, such as: B. Not being constantly bombarded by large asteroids and having a fairly stable axis of rotation. This is partly thanks to the planet Jupiter. The giant planet has helped clear the solar system of asteroid debris and may have helped stabilize the orbits of the inner planets. So life is good. But a new study shows life could be even better if Jupiter had a different orbit.
The study looked at how Jupiter affects Earth’s orbit and axial tilt over time. Both are important factors for our weather and 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 comes to 1. Earth’s orbit has an e = 0.017. This means that the earth is slightly closer to the sun for a little less than half of the year and a little further away for a little 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 point out that this eccentricity is not why we have seasons, and they would be mostly 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 sky in 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, it is summer when the earth is a little farther from the sun, and that means summers in the north are only slightly longer than summers in the south. Since much of the Earth’s landmass is in the Northern Hemisphere, this means that the Earth is slightly more livable than if we had a circular orbit. It doesn’t affect our daily lives, but it does matter on a geological level.
Over time, the Earth’s axial tilt changes slightly, resulting in more extreme or milder winters. The Earth’s axis also precesses over time, meaning that its orientation also shifts relative to the elliptical shape of Earth’s orbit. All of these factors underlie the so-called Milankovitch cycle. This is a measure of how much total heat the Earth’s surface receives, known as insolation. It varies over thousands of years. Twelve thousand years ago, at the dawn of human civilization, isolation was very high, giving us a particularly comfortable world. It is currently slightly 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. However, 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 was more eccentric. They thought a more eccentric Jupiter would make Earth less habitable, but were surprised it actually made things better. With an increased gravitational pull from Jupiter, Earth would have better insolation over its surface, so even more of Earth’s landmasses would be in a temperate region.
This has major implications for potentially habitable worlds in other star systems. Although we tend to focus on whether a world is within the habitable zone, that’s only the first requirement for a truly habitable world. Other factors such as solar radiation depend on the presence of other planets in the system. There is a gravitational dance between worlds that can affect or destroy a planet’s chances of life.
Relation: Vervoort, Pam, et al. “System Architecture and Planetary Skewness: Implications for Long-Term Habitability.” The Astronomical Journal 164.4 (2022): 130.