Could a collision with a wandering planet have brought water to Earth?

22 May 2019

Image: © Peter Jurik/

New research has found that the ancient planetary collision that formed the moon may have also brought water to Earth.

Given that we are the only planet so far known to support life, it is a given that Earth is pretty unique. However, what makes it particularly unique in the solar system is that it is the only planet with a large amount of water and a relatively large moon that stabilises the planetary axis.

Both of these were essential for life to have flourished here, but new research published to Nature Astronomy suggests that Earth got a massive help from a planet that came from a very distant part of the solar system.

Approximately 4.4bn years ago, the moon was likely formed after a planet named Theia – about the size of Mars – collided with Earth. It was theorised that Theia originated from the inner solar system. Now, researchers at the University of Münster have suggested it actually came from the outer solar system, and with it delivered large quantities of water to Earth.

The fact that Earth has water in the first place was always a bit of a mystery for scientists, given that it is located in the ‘dry’ inner solar system. Previous studies had shown that the solar system became structured so that ‘dry’ meteorites became separated from ‘wet’ meteorites. Also, it has been suggested that these carbonaceous, water-laden meteorites were responsible for bringing water to Earth.

However, it was unknown when and how this material arrived, at least until this latest study.

No life on Earth without the moon

“We have used molybdenum isotopes to answer this question. The molybdenum isotopes allow us to clearly distinguish carbonaceous and non-carbonaceous material, and as such represent a ‘genetic fingerprint’ of material from the outer and inner solar system,” said Dr Gerrit Budde, lead author of the study.

The researchers’ measurements showed that the molybdenum isotopic composition of the Earth lies between those of the carbonaceous and non-carbonaceous meteorites, demonstrating that some of Earth’s molybdenum originated in the outer solar system.

In doing so, they have shown for the first time that water-laden material arrived to Earth from the solar system late in the planet’s formation. Going a step further, they also said most of the molybdenum in the Earth’s mantle was supplied from the outer solar system by the planet Theia, which would have also originated in the outer solar system.

Thomas Kleine of the research team said of this discovery: “Our approach is unique because, for the first time, it allows us to associate the origin of water on Earth with the formation of the moon. To put it simply, without the moon there probably would be no life on Earth.”

Colm Gorey was a senior journalist with Silicon Republic