Solar system's edge may host newly spotted dwarf planet

Scientists have identified an object about 435 miles (700 kilometres) wide inhabiting the frigid outer reaches of our solar system that might qualify as a dwarf planet, spotting it as it travels on a highly elongated orbital path around the sun.

The researchers called it one of the most distant visible objects in our solar system and said its existence indicates that a vast expanse of space beyond the outermost planet Neptune and a region called the Kuiper Belt may not be deserted, as long thought.

The Kuiper Belt is populated by numerous icy bodies.

Given the name 2017 OF201, the object falls into a category called trans-Neptunian objects that orbit the sun at a distance beyond that of Neptune. The object takes about 25,000 years to complete a single orbit of the sun, compared to 365 days for Earth to do so.

The researchers said 2017 OF201 was identified in observations by telescopes in Chile and Hawaii spanning seven years.

"It is potentially large enough to qualify as a dwarf planet. Its orbit is very wide and eccentric, which means it experienced an interesting orbital migration path in the past," said astrophysicist Sihao Cheng of the Institute for Advanced Study in Princeton, New Jersey, who led the study with collaborators Jiaxuan Li and Eritas Yang, graduate students at Princeton University.

Its size is estimated to be a bit smaller than Ceres, which is the smallest of the solar system's five recognised dwarf planets and has a diameter of about 590 miles (950km). Pluto, the largest of those dwarf planets, has a diameter of about 1,477 miles (2,377km).

The mass of 2017 OF201 is estimated to be about 20,000 times smaller than Earth's and 50 times smaller than Pluto's.

"We don't know the shape yet. Unfortunately, it is too far away and it is a bit difficult to resolve it with telescopes," Cheng said. "Its composition is totally unknown yet, but likely similar to other icy bodies."

The discovery was announced by the Minor Planet Center of the International Astronomical Union, an international organisation of astronomers, and detailed in a study posted on the open-access research site arXiv. The study has not yet been peer-reviewed.

Earth's orbital distance from the sun is called an astronomical unit. 2017 OF201 is currently located at a distance of 90.5 astronomical units from the sun, meaning 90.5 times as far as Earth.

But at its furthest point during its orbit, 2017 OF201 is more than 1,600 astronomical units from the sun, while the closest point on its orbit is about 45 astronomical units. That means it sometimes is closer to the sun than Pluto, whose orbital distance ranges from 30 to 49 astronomical units as it travels an elliptical path around the sun.

The researchers suspect that the extreme orbit of 2017 OF201 may have been caused by a long-ago close encounter with the gravitational influence of a giant planet.

"We still don't know much about the solar system far away because currently, it is difficult to directly see things beyond about 150 astronomical units," Cheng said.

"The presence of this single object suggests that there could be another hundred or so other objects with similar orbit and size. They are just too far away to be detectable right now."

The five dwarf planets recognised by the International Astronomical Union are, in order of distance from the sun: Ceres, which is the largest object in the asteroid belt between Mars and Jupiter, then Pluto, Haumea, Makemake and Eris, which all orbit beyond Neptune.

The organisation defines a planet and a dwarf planet differently. A planet must orbit its host star — in our case the sun — and must be mostly round and sufficiently large that its gravitational strength clears away any other objects of similar size near its orbit. A dwarf planet must orbit the sun and be mostly round but it has not cleared its orbit of other objects.

Cheng said the discovery of 2017 OF201 has implications for hypotheses involving the potential existence of a ninth planet in our solar system, dubbed Planet X or Planet Nine.

This is because 2017 OF201's orbit does not follow the pattern exhibited by other known trans-Neptunian objects, which tend to cluster together. Some scientists had hypothesised that such clustering was caused by the gravity of a yet-to-be-discovered planet.

"The existence of 2017 OF201 as an outlier to such clustering could potentially challenge this hypothesis," Cheng said.