Paris - Sky-watchers reported on Wednesday that a small planet in deep space that triggered one of the fiercest controversies in modern astronomy appears to be a colder "twin" of Pluto.

The study, published in the journal Nature, is the biggest probe into the enigmatic planet known as Eris, whose discovery in 2005 raised questions about the Kuiper Belt, a zone of icy objects beyond the orbit of Neptune.

Eris stunned astronomers. It was initially estimated to be as big as Pluto, which had been enshrined as the Solar System's smallest, outermost planet after discovery in 1930.

The existence of Eris implied that scores, maybe even hundreds, more planets were just waiting to be spotted in the Kuiper Belt.

But were these objects - even Pluto itself - big enough to be considered real planets?

Contested

The question was answered in 2006, when Pluto was relegated by the International Astronomical Union (IAU) to a new category of "dwarf planet", where it has been joined by Eris, the big asteroid Ceres and two other large Kuiper belt objects, Makemake and Haumea.

Pluto's downsizing was unpopular with the public and is hotly contested even today by many astronomers, thus making Eris' name - after the Greek goddess of strife and discord - very apt.

The new probe used two giant telescopes in Chile's Atacama desert, which observed Eris as it passed in front of a star in November 2010, gaining clues about its size and surface from the distorted starlight.

This is a technical feat, for Eris was nearly 100 times as distant from Earth as Earth is from the Sun. It is the most distant object in the Solar System to be successfully observed this way.

The tableau of Eris that emerges is of a sphere-shaped object with a diameter of 2 326km, plus or minus 12km, which is a fraction smaller than earlier measurements.

In size terms, it is strangely a near-double of Pluto, whose diameter is estimated at 2 300 - 2 400km.

The surface of Eris is unusually bright, which suggests that it has an icy covering that is somehow refreshed. If the surface were permanently like this, it would become darkened by cosmic rays and impacts by micro-meteorites over time.

Temporary atmosphere

The theory is that Eris has a methane-rich atmosphere that in the depths of space freezes to the surface, but occasionally revives and then freezes again.

When the small planet reaches the closest part of its elliptical orbit around the Sun - a "mere" 30 astronomical units (AUs), or 30 times the distance between the Earth and the Sun - its frozen surface warms just enough to become gassy and create a thin but temporary atmosphere.

As it heads once more away from the Sun, the atmosphere freezes once more, clinging to the surface, according to this scenario.

"In that case, Eris would currently be a dormant Pluto twin, with a bright icy surface created by a collapsed atmosphere," suggests the paper, headed by Bruno Sicardy of the Pierre et Marie Curie University and Observatory of Paris.

Eris has a satellite, Dysnomia, named after the goddess' offspring and derived from the ancient Greek for lawlessness.

Mother and daughter take half a millennium - 557 years - to crawl around the Sun. Pluto and its moonlets complete their trip in a relatively brisk 248 years.

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