Timothy Birdnow

Theories about how the planets formed may be wrong - perhaps very wrong.

New research based on data sent back by the New Horizons probe indicate that the body known as Arrokoth (in the Kuiper Belt) formed from two distinct bodies. But, unlike the standard theory that says larger planetoids are formed from violent collisions, Arrokoth appears to have been gently created.

From the Science Alerts article:

Arrokoth, formerly known as (486958) 2014 MU69or Ultima Thule,was visited by the New Horizons probe on New Year's Day last year, way out in the Kuiper Belt.

At a staggering average distance from the Sun of 6.7 billion kilometres (4.1 billion miles), and an orbital period of 293 years, Arrokoth is the most distant single object in the Solar System we've identified.

The first of the three papers, by New Horizons investigator William McKinnon of Washington University in St. Louis and colleagues, found that the two objects in the Arrokoth binary actually formed close together.

There are generally two competing theories about how planets are born.

According to the longstandinghierarchical accretion modelof planetesimal formation, the building blocks of planets are formed when different parts of the solar nebula - the cloud of gas and dust that formed the Sun and planets - violently crash together.

On the other hand, thepebble accretion modelsuggests that elements from the same area gradually and gently come together to form binary objects.

The latest data from Arrokoth lends weight to the latter.

If Arrokoth had formed from chunks coming together from different parts of the nebula, it would have shown more evidence of impacts, the researchers said.

The second paper helped back this up, with astronomer John Spencer of the Southwest Research Institute and colleagues studying the surface of Arrokoth. They confirmed that it was smooth and only lightly cratered, a stark difference from other objects in the Solar System.

They also confirmed that Arrokoth has no rings or satellites larger than 180 metres (590 feet) within a radius of 8,000 kilometres, and no atmosphere, or gas or dust emission, the presence of which would indicate relatively recent perturbation. That indicates that Arrokoth has been pretty peaceful for a very long time indeed.

But they also studied Arrokoth's craters more closely, and found that the object's surface is likely around 4 billion years old - nearly as old as the Solar System itself.

"Arrokoth's spin state is likely to have evolved only very slowly, there do not appear to be sufficient impacts to act as effective seismic sources, and Arrokoth's likely high porosity would make seismic energy propagation highly inefficient," they wrote in their paper.

"Overall, despite the paucity of craters on its surface, the observed crater density is consistent with a crater retention age of greater than ~4 billion years. The visible surface at the scale of the LORRI image resolution thus plausibly dates from the end of Solar System accretion."

Finally, in the third paper, astronomer Will Grundy of Lowell Observatory and colleagues studied Arrokoth's peculiar redness. The reddest naturally occurring material - called "ultrared matter" - in the Solar System can be found in the Kuiper Belt, and Arrokoth is coated in it, but the material's exact nature was unclear.

The team found that the object is uniformly cold and red, coated in methanol ice and complex organic molecules they couldn't precisely identify based on the limited spectral data New Horizons was able to gather. These molecules are likely what creates the red colour.

This not only seems to confirm organic molecules as the source of ultrared matter; the uniformity of the colour - as well as the age of the surface as found by Spencer's team - also support the finding that Arrokoth was formed in a highly localised region.

"Arrokoth has the physical features of a body that came together slowly, with 'local' materials in the solar nebula," said Grundy. "An object like Arrokoth wouldn't have formed, or look the way it does, in a more chaotic accretion environment."

Posted by: Timothy Birdnow at 11:23 AM | Comments (43) | Add Comment
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