No exomoons yet, but we may have spotted a disk that will form them

No exomoons yet, but we may have spotted a disk that will form them

Enlarge / There’s a lot going on near a young star, with at least two planets forming around it.

Up until the last few decades, our picture of what might reside around distant stars was shaped entirely by the planets, moons, asteroids, and other bodies in our own Solar System. But the discovery of thousands of exoplanets has dramatically improved our picture of what’s out there in terms of large bodies. Comets and asteroids, by contrast, are well below our ability to image for the indefinite future.

Moons, however, are awkwardly in between. It should be possible to image them indirectly, as their gravitational influence will alter the timing with which their planets orbit the star. And we might get a more direct indication of their presence as they will sometimes add to the shadow cast as transiting planets pass in front of their host star. We’ve searched for these

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China’s Chang’E-4 may have landed near pieces of the Moon’s interior

China’s Chang’E-4 may have landed near pieces of the Moon’s interior

Enlarge / The original Yutu rover, shown on the Moon. (credit: NASA)

While some of the details are still being worked out, it’s generally agreed that the Moon formed when a Mars-sized body collided with the early Earth. Some of the debris put into orbit by the collision would then go on to condense into the Moon.

One of the consequences of this is that the early Moon spent a lot of its history being bombarded by this debris, a process that should have left its surface molten. This magma ocean would only solidify slowly as the bombardment wound down, and the process of solidification should have left a mark on the Moon’s composition. So far, indications of this have been difficult to come by. But now, there are signals that the Chang’E-4 mission to the Moon’s far side has finally spotted some of the Moon’s mantle, which

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First results from New Horizons’ time in the Kuiper Belt

First results from New Horizons’ time in the Kuiper Belt

Enlarge / When Ultima met Thule. A view of the two-lobed body, showing the bright neck and the large Maryland crater. (credit: NASA/Johns Hopkins University APL/Southwest Research Institute.)

For many at the Johns Hopkins Applied Physics Laboratory, January 1 this year didn’t mean a New Year’s celebration. Instead, it meant the first arrival of data from New Horizons’ visit to a small Kuiper Belt object. But, like its earlier flyby of Pluto, the probe was instructed to grab all the data it could and deal with getting it back to Earth later. The full set of everything New Horizons captured won’t be available for more than a year yet. But with 10 percent of the total cache in hand, researchers decided they had enough to do the first analysis of 2014 MU69.

2014 MU69 is thought to preserve material as it condensed in the earliest days

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Impact that formed the Moon might have splashed into Earth’s magma ocean

Impact that formed the Moon might have splashed into Earth’s magma ocean

Enlarge / An early version of the collision model, showing a head-on impact.

The Earth and its moon are unique in our Solar System. Earth is the only rocky planet with a large moon, and only the dwarf planet Pluto has a moon that’s so similar in size to its host planet. The Moon is also remarkably similar to the Earth in terms of its composition, suggesting they formed from the same pool of material instead of the Moon forming elsewhere and having been captured.

This collection of properties led to a number of ideas about how the Moon formed, all of which failed to fit the data in various ways. Eventually, however, scientists came up with an idea that seemed to get most of the big picture right: a collision between the Earth and a Mars-sized object happened early in the Solar System’s history, creating a cloud of debris

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