This Asteroid Shouldn’t Be Where Astronomers Found It

An artist's impression of 2004 EW95, a relic of the primordial solar system, which scientists believe originated in the asteroid belt between Mars and Jupiter but now floats beyond Neptune.

At first the astronomers thought it was a mistake. They had found a carbon-covered asteroid floating among countless icy bodies far away in our solar system. The newly discovered space rock, which they named 2004 EW95, was something the scientists would have expected to have seen in the asteroid belt between Mars and Jupiter. Instead, it was dancing near Neptune.

Their finding, which was published Wednesday, suggests that 2004 EW95 is the first of a new class of space objects lurking in the outer solar system, in a vast, frigid region known as the Kuiper belt that still contains many mysteries. The researchers believe that the asteroid sling-shotted from the inner solar system some 4.5 billion years ago, and that it may provide insight into the early formation of our planets.

Using the European Southern Observatory’s Very Large Telescope, Tom Seccull, a doctoral student at Queen’s University Belfast in Northern Ireland, and his colleagues examined light signatures from the icy surfaces of Kuiper belt objects. They noticed something strange about one object that measured nearly 190 miles long and was located 2.5 billion miles from Earth. Their analysis showed that this object did not share the same frigid past as the ice balls drifting nearby.

“When we first looked at this, we thought it was wrong,” said Mr. Seccull, who is lead author of the paper published in The Astrophysical Journal Letters. “The rock had been altered by the presence of liquid water.”

That most likely meant that the asteroid once resided in a hotter environment much closer to the sun. The team also found that the asteroid’s chemistry was dominated by compounds called ferric oxides and phyllosilicates, which had never been seen before on Kuiper belt objects.

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These pieces of evidence led the team to conclude that 2004 EW95 was formed in the inner solar system, and had most likely been hurled to the outer solar system as the giant gas planets, Jupiter and Saturn, migrated away from the sun.

One of the prevailing ideas in astronomy is that our solar system formed in chaos. Some 4.5 billion years ago, baby planets and discs of dust collided with one another violently over millions of years.

At some point, the gas giants moved from their positions in the inner solar system to the outer solar system. This has been called the “grand tack” hypothesis. The researchers aren’t sure how exactly or at what point 2004 EW95 was flung from the interior, but they think it coincided with the gas giant migration.

“It’s a misplaced witness to the early history of the formation of our planetary system and the sun,” said Thomas Puzia, an astronomer at Pontificia Universidad Católica de Chile and an author of the paper.

Jupiter’s big move was crucial to our existence, Dr. Puzia said. Across the galaxy, scientists have found evidence of gas giants that did not journey outward, away from their stars. These “hot Jupiters” often prevented smaller exoplanets from forming, a scenario that could have potentially happened here had Jupiter not scooched over.

By finding 2004 EW95, astronomers now have stronger evidence that objects near the sun, like asteroids and gas giants, moved into the outer solar system billions of years ago.

“We are lucky to be here, because the gas giants didn’t destroy the inner solar system,” said Dr. Puzia. “Understanding that — why it happened and how early it happened — tells us more about our birthplace.”

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