Skip to main content

How did the planets Uranus and Neptune become so different?

Uranus (left) and Neptune photographed by Voyager 2.
Uranus (left) and Neptune photographed by Voyager 2. NASA/JPL

In our solar system, the two outermost planets of Uranus and Neptune are something of an oddity. Although both planets are of a similar mass and are similarly distant from the sun, the two are markedly different in terms of the way that they orbit and how their moons and satellites move around them. A new paper proposes a theory for why this is: Because of two different massive impact events during their formation.

Despite the planets’ apparent similarity, “there are also striking differences between the two planets that require explanation,” Christian Reinhardt, lead author of the paper, said in a statement.

The first major difference between the two planets is their tilt. Like Earth, most planets in our solar system are slightly titled on their axes so one part of the planet is closer to the sun at different times of year, which is why we have seasons. Both Uranus and Neptune have extremely long seasons, lasting up to a decade.

But the notable difference is that Uranus is titled to an extreme degree, to such an extent that it is almost completely on its side. “Uranus and its major satellites are tilted about 97 degrees into the Solar plane and the planet effectively rotates retrograde with respect to the sun,” co-author Joachim Stadel said in the statement.

Another difference is the satellites of the planets. Both Uranus and its moons are titled at the same angle, which suggests they formed from a disk, like Earth’s moon. But on Neptune, its largest moon Triton orbits at an angle relative to the planet, suggesting that the moon was formed elsewhere and was pulled in by Neptune’s gravity.

The paper proposes that the differences can be explained by the way the planets formed. Both began in similar circumstances, but were differently affected by impacts.

The researchers ran computer simulations and found that the most likely cause of Uranus’ tilt is that it was grazed by a large object which pushed the planet over onto its side but did not affect its interior. In the case of Neptune, the planet seems to have been subject to a head-on collision which did affect its interior, leading to heat moving from the interior to the surface much more quickly than on other planets.

“We clearly show that an initially similar formation pathway to Uranus and Neptune can result in the dichotomy observed in the properties of these fascinating outer planets,” co-author Ravit Helled summarized.

The findings are published in the journal Monthly Notices of the Royal Astronomical Society.

Editors' Recommendations

Georgina Torbet
Georgina is the Digital Trends space writer, covering human space exploration, planetary science, and cosmology. She…
How Curiosity is keeping itself entertained on Mars over the long weekend
Curiosity Rover

It might be a long weekend in the U.S., but on Mars, the Curiosity rover doesn't get holidays off.

Typically, the Curiosity team will plan out all the rover's activities over the weekend, then leave the rover to execute those plans while they take a well-deserved weekend break. The time for the rover is planned out in "sols," or Martian solar days, which are just a touch longer than an Earth day, at 24 hours, 39 minutes, 35 seconds long.

Read more
NASA wants to send a spacecraft to Neptune’s strange moon Triton
This global color mosaic of Neptune's moon Triton was taken in 1989 by Voyager 2 during its flyby of the Neptune system.

NASA has proposed sending a spacecraft to Neptune's strange moon Triton, to learn more about this mysterious body.

The last and only time a spacecraft visited Triton was 30 years ago, when the Voyager 2 spacecraft flew past it on its way out of the solar system. This visit discovered some strange puzzles about the moon, showing that there were huge icy plumes coming off the surface, but scientists couldn't understand why. The finding suggested that there could still be some activity in this distant moon, and researchers want to know how this could be possible given that it is so far from the sun.

Read more
A lucky dip into Jupiter’s clouds captures stunning image of the planet
image showing the entire disk of Jupiter in infrared light

Astronomers have captured some of the highest ever resolution infrared images of Jupiter taken from the Earth, using the Gemini North telescope in Hawaii.

“The Gemini data were critical because they allowed us to probe deeply into Jupiter’s clouds on a regular schedule,” explained Michael Wong of University of California Berkeley, leader of the research team, in a statement. “We used a very powerful technique called lucky imaging."

Read more