An international team of astronomers led by Leiden University has discovered what may be a "toddler" exoplanet. Orbiting a young binary star system called CS Cha about 538 light years from Earth in the constellation of Chameleon, the object may be less than two million years old and is still growing thanks to the dust disc that it inhabits.

Astronomical discoveries aren't always eureka moments. Because telescopes are essentially gigantic cameras that spend their careers snapping pictures of the heavens, there's always a huge library of images to be gone over years or even decades after they were taken.

Such is the case of the Leiden team who were studying CS Cha and its dust disc using the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) on the European Very Large Telescope in Chile. At only about two or three million years old, the T Tauri-type binary star system is very young and consists of two very small stars less than 30.6 AU (4.6 billion km, 2.8 billion miles) apart. These stars orbit a common center of gravity, and are surrounded by a disc of dust and gas about 55 AU across.

What they did not expect to see was a faint companion object orbiting the two stars at a distance of about 210 AU. To confirm that this was actually a companion and to find out more about it, the team went through the image archives from the Very Large Telescopegoing back 11 years, as well as 19-year-old images collected by NASA's Hubble Space Telescope.

Exactly what the companion is has yet to be determined, but the team claims that, based on various models, it is either a super-Jupiter exoplanet in the very early stages of its development or a small brown dwarf star.

"The most exciting part is that the light of the companion is highly polarized," says team leader Christian Ginski of the Leiden Observatory at Leiden University. "Such a preference in the direction of polarization usually occurs when light is scattered along the way. We suspect that the companion is surrounded by his own dust disc. The tricky part is that the disc blocks a large part of the light and that is why we can hardly determine the mass of the companion. So it could be a brown dwarf but also a super-Jupiter in his toddler years. The classical planet-forming-models can't help us."

To gain a more definitive answer as to the companion's nature, the team plans to make a more detailed study using the international ALMA telescope on the Chajnantor plateau in the North Chilean Andes.

The research was published in Astronomy and Astrophysics.

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