New Candidate for Coldest Known Star May Help Blur the Distinction Between a Star and a Planet

An artist?s impression of the CFBDSIR 1458+10 star system. The star in the background is the candidate for the coldest known star. Image: ESO/L. Calçada

There may be a new candidate for the coldest known star, an object classified as a brown dwarf with a surface temperature of only about 100 degrees Celsius. The temperature is close to the boiling point of water which is hot when compared with temperatures obtained on the surface of the Earth. In cosmic terms, however, this temperature is remarkably cold, in fact about 55 times colder than the surface of the Sun. Yet, the title for the coldest star rests on an important distinction: is this particular object a star, and can the definition of a star actually change?

This particular cool object, named CFBDSIR 1458+10B, is part of a binary star system at a distance of about 75 light-years from the Earth. Both parts of the binary system are classified as brown dwarfs¹, and are about the size of Jupiter. They orbit each other at a separation distance of about 3 AU, or 3 times the distance from the Earth to the Sun, in a time period lasting 30 years. The system was detected last year by the Very Large Telescope, part of the European Souther Observatory in Chile. At first it was thought to be a single star with a very low temperature², but the presence of a smaller even colder companion made it much more interesting.

A color image of the star system made using four different filters at near-infrared wavelengths. Captured at the Keck II Telescope in Hawaii. Image: Michael Liu, University of Hawaii.

Brown dwarfs are often considered failed stars, and are objects that are not able to gather enough mass so that their gravity can trigger the Hydrogen-1 fusion reaction that takes place in the cores of most other stars. Smaller brown dwarfs, due to their low mass, begin to more closely resemble gas giant planets. There is still a lingering question on whether or not this object can, or even should be considered a brown dwarf and a star. The most simple distinction between stars and planets is whether or not there is any fusion taking place inside the star. By this definition, the cutoff point occurs at about 13 times the mass of Jupiter. At this mass, objects start to fuse deuterium and can be considered stars. Since the mass of this object is between 6 and 15 times the mass of Jupiter, there is a possibility that nuclear fusion is not happening. There is a new argument brought up by some astronomers that the formation of an object should factor into whether it can be a classified as a star or not, which is much more difficult to measure and determine. Brown dwarfs, and other stars as well, are formed due to the collapse of interstellar gas and dust. Planets on the other hand are thought to be formed as a disc of dust accretes into distinct bodies around a star. CFBDSIR 1458+10B is thought to be formed through the first process, and it is argued that it should subsequently be thought of as a star.

This candidate for the coldest star known is interesting in that it represents a potential shift in defining the lower mass limit for stars. What particularly excites me about this discovery, though, is not the change in classification that it might bring but that it is a new example of a type of object that we do not understand very well yet. This object lies very close to the boundary between a large gas giant planet and a brown dwarf star, and subsequently exhibits properties from both types of objects, especially in its atmosphere. Hot brown dwarf stars obtain their color mostly through the presence of sodium and potassium atoms in their atmospheres. In cooler brown dwarfs, and likely in an object like this one, it is expected that the sodium and potassium atoms join together into molecules, like potassium chloride, and are subsequently removed from the atmosphere. Due to this effect, their colors should be different. There is also the possibility of the presence of water clouds in the atmosphere, much more typical of gas giants. Regardless of whether or not this object is a star, it is undoubtedly very interesting.

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