巨大的行星撞击木星的年轻人

This week scientists studying our Solar system’s iconic gas giant, Jupiter, think they’ve found evidence that something ten times the size of the Earth slammed into the planet way back in history, reconfiguring the shape of its core. As Izzie Clarke has been finding out.

Izzie - Jupiter is the biggest planet in our solar system, and it’s atmosphere is mostly made of hydrogen gas and helium gas - that’s why it’s called a gas giant. But this planet’s formation and structure has been a bit of a mystery…

Ravit - We thought that Jupiter could have a core, with the core being quite compact; a few tens of percent of the planetary radius. The composition is primary heavy elements so that means metals, rocks and ices

Izzie - That’s Ravit Helled from the University of Zurich in Switzerland, who has been using computer models to explore how the gas giant might’ve formed and whether this core really existed. Eight years ago, in August 2011, NASA launched Juno, a three pronged, almost claw-like probe to do some exploring…

Ravit - The aim of the mission was to really understand Jupiter as a planet, looking at different aspects. The gravity field, the magnetic field, the atmospheric variations… And by that better understand the origin of our own solar system and also better understand giant planets in general.

Izzie - Juno reached Jupiter three years ago and the mission is only halfway through. But Ravit has been interested in Jupiter’s gravity…

Ravit - The gravity measurements of Juno are essentially used to make new structure models of the planet. We can understand what the planet looks like from the inside; what is it made of, and how the material is distributed within the planet. New structure models of Jupiter that fit Juno data essentially tell us that the core of Jupiter might be fuzzy or diluted and rather large. Which is very different than this standard compact core that we had in mind before, before the Juno mission.

Izzie - I see. It's actually not like a little compact thing in the middle and then something else completely surrounding it. It's more diluted. How can that be possible?

Ravit - Exactly. It means that the core can be extended to a few tens of percent of the planet's radius. And it also means that the core is not very distinct within the planet. It can also consist of lighter material so it's not just pure heavy elements but it could also be mixed with some hydrogen and helium.

It can also be that you know it's distributed in a gradual way so you don't have a distinct core-envelope boundary. You don't just have the hydrogen and helium envelope above it but you somehow have a more gradual transition between the inner part of the planet and the outer part of the planet.

Izzie - What do you think could cause that? What would cause that?

Ravit - Yeah we didn't expect that. So we think it was a giant impact of a large planetary embryo that hit Jupiter right after its formation.

Ravit - What could that have been?

Ravit - If you have a giant impact which means that you have a huge body impacting Jupiter. In our simulations, that the body was assumed to be 10 Earth masses, so it's a very large object. And then if it impacts Jupiter in the right velocity and geometry, so it really comes head on, it can really penetrate all the way to the centre of the planet. Basically, it can destroy the core and lift the material to the outer part of the planet and then create this dilute core.

After the impact, we modelled the evolution of the planet and we asked “can we get something that looks like Jupiter today?” And the answer was yes.

Izzie - But we’ve seen this before. Impacts are important to understand other planets in our solar system; on mercury there’s a high metal-to-rock ratio, a lot of planets are tilted most likely due to impacts, and our own moon is probably a result of one. And as technology and space exploration advances, this insight into jupiter can help those discovering planets outside our solar system….

Ravit - I think we are in a great time now to do this kind of research because we can really explore the solar system planets in detail and at the same time have this huge great statistics of planets around other stars. The idea is to bridge these two aspects and get a better understanding of giant planets and planets in general.