5 August 2014

An international team of astronomers, led by the University of Hertfordshire, has discovered an extremely cool object that could have a particularly diverse history - although it is now as cool as a planet, it may have spent much of its youth as hot as a star.

The current temperature of the object is 100 to 150 degrees Celsius, an intermediate temperature between that of the Earth and Venus. However, 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, following the other classes O, B, A, F, G, K, M, L and T. Although the temperature is similar to that of the planets, the object is not like the rocky Earth-like planets.  Instead it is a giant ball of gas like Jupiter.

The international team, led by Professor David Pinfield from the Centre for Astrophysics Research at the University of Hertfordshire, identified the Y dwarf using the WISE (Wide-field Infrared Survey Explorer) observatory - a NASA space telescope that has imaged the entire sky in the mid-infrared wavelength. The team also measured the spectrum of light emitted by the Y dwarf, which allowed them to determine its current temperature and better understand its history. Their work is published in the Monthly Notices of the Royal Astronomical Society.

Only twenty 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.

Professor Pinfield explained: "Our measurements suggest that this Y dwarf may have a composition and/or age characteristic of one of the Galaxy's older members. This would mean its temperature evolution could have been rather extreme."

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

If WISE J0304-2705 is an ancient object, then its temperature evolution would have followed through an understood series of stages (as shown in the illustration).  During its first approximate 20 million years it would have a temperature of at least 2800 degrees Celsius, the same as red dwarf stars like Proxima Centauri (the nearest star to our Sun). After 100 million years it would have cooled to about 1500 degrees Celsius, with silicate clouds condensing out in its atmosphere. At a billion years of age it would have cooled to about 1000 degrees Celsius, so cool that methane gas and water vapour would dominate its appearance. Since then it would have continued to cool to its current temperature, barely enough to boil water for a cup of tea.

WISE J0304-2705 is as massive as 20-30 Jupiters combined, which is intermediate between the more 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, Professor Pinfield's team made crucial ground-based observations with some of the world's largest telescopes - the 8 metre Gemini South Telescope, the 6.5 metre Magellan Telescope and the European Southern Observatory's 3.6 metre 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."

Professor Maria Teresa Ruiz, team member from the Universidad de Chile, remarked: ."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."

WISE J0304-2705 is located in the Fornax (Furnace) constellation, belying its cool temperature. There is currently no lower limit for Y dwarf temperatures, and there could be many even cooler and more diverse objects undetected in the solar neighbourhood.

The WISE observatory 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 three year mission extension.

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