Planet-like object may have spent its youth as hot as a star

The current temperature of the object is 100-150 degrees Celsius, intermediate between that of Earth and Venus. But the object shows evidence of a possible ancient origin, implying that a large change in temperature has taken place. In the past this object would have been as hot as a star for many millions of years.

Called WISE J0304-2705, the object is a member of the recently established "Y dwarf" class -- the coolest stellar temperature class yet defined, added to the end of the sequence OBAFGKMLT (for historical reasons this is not in alphabetical order but follows a decline in temperature from O to T). Although its temperature is not far off that of our own world, the object is not like the rocky Earth-like planets and instead is a giant ball of gas like Jupiter.

The international discovery team, led by Prof David Pinfield at the University of Hertfordshire, identified the Y dwarf using the Wide-field Infrared Survey Explorer (WISE) observatory -- a NASA space telescope that since its launch in 2009 has imaged the entire sky in mid-infrared light (rather redder than the reddest light we can see with our eyes). The team also dispersed the light emitted by the Y dwarf into a spectrum, which allowed them to determine its current temperature and better understand its history.

Only 20 other Y dwarfs have been discovered to-date, and amongst these WISE J0304-2705 is defined as 'peculiar' due to unusual features in its emitted light spectrum. "Our measurements suggest that this Y dwarf may have a composition and/or age characteristic of one of the Galaxy's older members" explains Prof Pinfield. "This would mean its temperature evolution could have been rather extreme -- despite starting out at thousands of degrees this exotic object is now barely hot enough to boil a cup of tea."

The reason that WISE J0304-2705 underwent such extensive evolutionary cooling is because it is "sub-stellar" -- its interior never became hot enough for hydrogen fusion, the process that has kept the Sun hot for billions of years. And without an energy source maintaining a stable temperature, cooling and fading was inevitable.

If WISE J0304-2705 is an ancient object then its temperature evolution would have followed the stages shown in the illustration. During the first 20 million years or so of its life it would have had a temperature of at least 2800 degrees C, the same as red dwarf stars like Proxima Centauri (the nearest star to the Sun). After 100 million years it would have cooled to about 1500 degrees C, with silicate clouds condensing out in its atmosphere. At a billion years of age it would have cooled to about 1000 degrees C, cool enough for methane gas and water vapour to dominate its appearance. And since then it has continued to cool to its current temperature of 100 to 150 degrees C.

WISE J0304-2705 is as massive as 20 to 30 Jupiters combined, so somewhere between the least massive stars and typical planets. But in terms of temperature it may have actually "taken the journey" from star-like to planet-like conditions.

Having identified WISE 0304-2705, Prof Pinfield's team made crucial ground-based observations with some of the world's largest telescopes -- the 8m Gemini South Telescope, the 6.5m Magellan Telescope and the European Southern Observatory's 3.6m New Technology Telescope, all located in the Chilean Andes.

Team member Dr Mariusz Gromadzki said "The ground based measurements were very challenging, even with the largest telescopes. It was exciting when the results showed just how cool this object was, and that it was unusual."

"The discovery of WISE J0304-2705, with its peculiar light spectrum, poses ongoing challenges for the most powerful modern telescopes that are being used for its detailed study" remarked Prof Maria Teresa Ruiz, team member from the Universidad de Chile.

WISE J0304-2705 is located in the Fornax (Furnace) constellation in the southern hemisphere of the sky, belying its cool temperature and is between 33 and 55 light years away.

There is currently no lower limit for Y dwarf temperatures, and there could be many even cooler and more diverse objects un-detected in the solar neighbourhood. WISE went into hibernation in February 2011 after carrying out its main survey mission. However, by popular demand it was revived in December 2013, and is continuing to observe as part of a 3 year mission extension.

"WISE gives us wonderful sensitivity to the coolest objects" said Prof Pinfield, "and with 3 more years of observations we will be able to search the sky for more Y dwarfs, and more diverse Y dwarfs."