Closing in on a giant ringed planet

The mystery of an exoplanet with a ring system more than 100 times larger than the one that girdles Saturn will not be resolved until at least 2021, evidence suggests.

Currently, the existence of the exoplanet, dubbed J1407b, is only hypothetical – a speculative answer to the question of why in 2007 a star, J140747.93−394542.6, or J1407 for short, 460 light-years from Earth in the constellation of Centaurus, appeared to undergo “a series of symmetric, deep eclipsing events”. 

The ostensible eclipses – detected as a periodic dimming of the star’s light – occurred over a 52-day period. However, only one of them was observed in detail, limiting, thus, the amount of information that could be gleaned. A favoured, if tentative, explanation for the phenomenon was that light from J1407 was temporarily obscured (from the point of view of Earth observers) by an object surrounded by a vast array of rings. 

In 2012, American Association of Variable Star Observers (AAVSO) relayed a request from Eric Mamajek of the Cerro Tololo Interamerican Observatory in Chile for assistance in monitoring the star in the hope of seeing another fade-out.{%recommended 2013%}

The suggested “ringed companion”, wrote the AAVSO’s Elizabeth Waagen, was “likely to be a brown dwarf or giant planet” that “may constitute a moon-forming ‘protoexosatellite disk’.”

Thus far, however, the world’s astronomers have come up with bupkis. 

Now Robin Mentel of Leiden University in the Netherlands has come up with at least partial evidence to explain why.

In a paper published in the journal Astronomy and Astrophysics, he and colleagues detail a novel investigation that involved looking to the past in order to constrain possibilities in the future.

Over a period of two years, Mentel studied historical photographic plates showing the star, comparing its brightness with that of other, known, nearby stars to see if any dimming – and thus a likely eclipse – could be seen. 

By the end of the investigation, 490 plates had been studied, the oldest dating to 1890.

And the result? Once again, bupkis. In every shot, J1407 shone fine and bright, its intensity never wavering.

This, however, Mentel and colleagues are quick to point out, does not mean that there wasn’t an eclipse in the 107 years between the earliest image being recorded and the 2007 dimming being observed – after all, the plate collection did not constitute a continuous, unbroken record – but it does make it possible to suggest how much time elapses between events.

On that basis, Mentel suggests that the ringed planet – or ringed brown dwarf – will transit the star again in either 2021 and 2024.

Providing, of course, that it exists at all. One thing, however, is sure: In three years, the hunt for the possible “protoexosatellite disk” will kick up a gear.