Varying degrees of orbital eccentricity around a central star. (NASA/JPL-Caltech)
As scientists work to understand what might make a distant planet habitable, one factor that is getting attention is the shape of the planet’s orbit, how “eccentric” it might be.
It might seem that a perfect circular orbit would be ideal for habitability because it would provide stability, but a new model suggests that it is not necessarily the case. The planet in question is our own and what the model shows is that if Jupiter’s orbit were to change in certain ways, our planet might become more fertile than it is.
The logic play out as follows:
When a planet has a perfectly circular orbit around its star, the distance between the star and the planet never changes and neither does the in-coming heat. But most planets — including our own — have eccentric orbits around their stars, making the orbits oval-shaped. When the planet gets closer to its star it receives more heat, affecting the climate.
Using multi-factored models based on data from the solar system as it is known today, University of California, Riverside (UCR) researchers created an alternative solar system. In this theoretical system, they found that if Jupiter’s orbit were to become more eccentric, it would in turn produce big changes in the shape of Earth’s orbit. Potentially for the better.
“If Jupiter’s position remained the same but the shape of its orbit changed, it could actually increase this planet’s habitability,” said Pam Vervoort, UCR Earth and planetary scientist and study lead author.
The paper upends two long-held scientific assumptions about our solar system, she said.
“Many are convinced that Earth is the epitome of a habitable planet and that any change in Jupiter’s orbit, being the massive planet it is, could only be bad for Earth,” Vervoort said in a release. “We show that both assumptions are wrong.”
Size comparison of Jupiter and Earth shows why any changes relating to the giant planet would have ripple effects. (NASA)
As she and colleagues report in the Astronomical Journal, if Jupiter pushed Earth’s orbit to become more eccentric based on its new gravitational pull, parts of the Earth would sometimes get closer to the sun. As a results, parts of the Earth’s surface that are now sub-freezing would get warmer, increasing temperatures in the habitable range.
While the Earth-Jupiter connection is a focus of the paper and forms a relationship that’s not hard to understand, the thrust of the paper is modeling how similar kinds of exoplanet orbits and solar system relationships can affect habitability and the potential for life to emerge and prosper.
“The first thing people look for in an exoplanet search is the habitable zone, the distance between a star and a planet to see if there’s enough energy for liquid water on the planet’s surface,” said Stephen Kane, UCR astrophysicist and study co-author.
During its orbit, different parts of a planet receive more or fewer direct rays, resulting in the planet having seasons. Parts of the planet may be pleasant during one season, and extremely hot or cold in another.
“Having water on its surface a very simple first metric,” he said, in the release. But “it doesn’t account for the shape of a planet’s orbit, or seasonal variations a planet might experience.”
A habitable zone, shown in green here, is defined as the region around a star where liquid water, an essential ingredient for life as we know it, could potentially be present. But planets can go in and out of the habitable zone during their lifetimes, depending on changing shapes of their orbits based on the movements of other planets in the system. (NASA-JPL/Caltech)
Existing telescopes are capable of measuring a planet’s orbit. However, there are additional factors that could affect habitability, such as the degree to which a planet is tilted toward or away from a star. The part of the planet tilted away from the star would get less energy, causing it to be colder.
Pam Vervoort is a teaching assistant and doctoral student at the University of California, Riverside.
This same study found that if Jupiter were positioned much closer to the Sun, it would induce extreme tilting on Earth, which would make large sections of the Earth’s surface sub-freezing.
It is more difficult to measure tilt, or a planet’s mass, so the researchers would like to work toward methods that help them estimate those factors as well.
Ultimately, the movement of a giant planet is important in the quest to make predictions about the habitability of planets in other systems as well as the quest to understand its influence in this solar system.
“It’s important to understand the impact that Jupiter has had on Earth’s climate through time, how its effect on our orbit has changed us in the past, and how it might change us once again in the future,” Kane said.
And what we learn about how Jupiter’s orbit influences Earth’s orbit will not only teach us about our solar system but teach about the potential for life on some of the billions of other solar system planets out there.
The paper concludes as follows:
“The era of exoplanet exploration and characterization has only just begun. Many of the parameters required to simulate reliably the orbital and spin dynamics of habitable zone exoplanets …. are currently unobtainable” — such as the speed of a planet’s rotation, the initial tilt of its axis, continuous changes in the orientation of the body’s rotational axis, and the eccentricity of its orbit.
“However, it is expected that significant advances will be made in observational facilities and analysis techniques in the coming years. This, combined with our growing understanding of the evolution of protoplanetary disks and planet formation, will undoubtedly provide us with a plethora of new tools to improve exoplanet characterization, and provide the critical data needed to more reliably assess the potential habitability of newly discovered exoplanets.”
