Earth is not only habitable, it is extraordinarily habitable. It’s fairly wet for a planet so close to the sun, it’s geologically active, and it has a stable orbit, all of which are essential to 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 spindle. This is partly due to 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 good. But a new study shows that if Jupiter had a different orbit, life could be better.
The study looked at the way Jupiter affects Earth’s orbit and axial tilt over time. Both are important factors in 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 deflection. At e = 0, the orbit is a full circle, and the more elliptical the orbit, the closer e is to 1. Earth’s orbit is e = 0.017. This means that the Earth is a little closer to the sun for a little less than half of the year, and a little farther for a little more than half. This is because the Earth moves slightly faster in its orbit when it is closer to the Sun rather than further away.
At this point, skilled readers will be keen to point out that this deviation is not the reason for chapters to exist, and they will often be right. The reason for our usual seasons is due to the axial tilt of the Earth, which is currently about 23.4 degrees. Because of this tilt, the sun is higher in the summer, and lower in the sky during the winter. And of course, when it’s summer in the northern hemisphere, it’s winter in the southern hemisphere.
Interestingly, for the Northern Hemisphere, it is in the summer when the Earth is a little further from the sun, meaning that the northern summer is slightly longer than the southern summer. Since most of the land mass on Earth is in the Northern Hemisphere, this means that the Earth is slightly more habitable than if we had a circular orbit. It does not affect our daily lives, but it does play a role on geological scales.
Over time, the axial tilt of the Earth changes slightly, giving more severe or mild winters. The Earth’s axis also advances over time, which means that its orientation relative to the elliptical shape of Earth’s orbit also changes. All these factors are behind what is known as Milankovitch cycle. This is a measure of the total amount of 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, the insulation was very high, which gave us a particularly interesting world. At the moment, it is somewhat lower, and without the effects of global warming, we would be in a cold 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 that a more exotic Jupiter would make Earth less habitable, but were surprised to find that it actually makes things better. With an increase in the gravitational influence of Jupiter, the Earth would have better insolation on its surface, so that more land masses on Earth would be within the moderate range.
This has major implications for viable worlds in other star systems. Although we tend to focus on whether the world is within the habitable zone, that is 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 chances of life.
Reference: Verfort, Pam, et al”.System structure and planetary aberration: implications for long-term habitation. ” Astronomical Journal 164.4 (2022): 130.