Brown Dwarf ‘The Accident’: Neither Star Nor Planet

Brown Dwarf

Brown dwarfs are sub-stellar objects with a mass that ranges between that of a planet and a star. Brown dwarfs are considered sub-stellar objects as they lack the mass required to satisfy nuclear fusion to become fully-fledged stars. Hence dubbed as “failed stars” by many, ” brown dwarfs emit dim light due to thermal radiation. However, this one peculiar brown dwarf that popped into view and was discovered by accident puzzles the astronomers due to its out of the-brown-dwarf-ordinary characteristics.

Found ‘The Accident’ by Accident

Dan Caselden, a citizen scientist came across this bizarre brown dwarf inadvertently while he was running an automated search on his online program he built to find brown dwarfs in NASA NEOWISE (Near Earth Objects Wide-field Infrared Survey Explorer) space telescope images. His program could rule out stationary cosmic objects like distant stars that emit infrared light and locate the ones that are moving with characteristics similar to the discovered brown dwarfs. An object was flitting across the screen glooming faintly and that was when he made this chance discovery. Conventionally known as WISEA J153429.75-1043303.3, this peculiar brown dwarf earned the nickname “The Accident” as it was discovered by accident. The Accident deviates from the typical discovered brown dwarfs scattered in the galaxy to date due to its singular characteristics.

The Accident discovered by Dan Caselden, a citizen scientist.
Figure 01: The dark spot moving in the bottom left corner is the brown dwarf, The Accident discovered by Dan Caselden, a citizen scientist.
Credits: NASA/ JPL-Caltech/ Dan Caselden

This object defied all our expectations… This discovery is telling us that there’s more variety in brown dwarf compositions than we’ve seen so far. There are likely more weird ones out there, and we need to think about how to look for them.

Davy Kirkpatrick, an astrophysicist, IPAC at Caltech in Pasadena, California

What are Brown Dwarfs?

Predominantly speaking, the astronomers allocate cosmic objects with between 15 times the mass of Jupiter and 75 times the mass of Jupiter to be owned by the domain of brown dwarfs.

Comparison of planets, exoplanets, brown dwarfs and stars.
Figure 02: Brown dwarfs have specific characteristics of both planets and stars. Credits: NASA/ JPL-Caltech

Invading the gap between stars and planets in the ladder, brown dwarfs form on their own like stellar objects in the beginning. Brown dwarfs are star-like and do not fit into the category ‘stars’ as they lack the necessary mass to kickstart fusion in their cores. Nuclear fusion is the process which fuses together protons through a series of steps to form Helium occurring inside the core of the stars. Four Hydrogen nuclei are slammed together forming a Helium nucleus and this Helium nucleus weighs slightly less than the four Hydrogen nuclei combined. Thus, this mass difference is transformed into energy given by the mass-energy equivalence (E = mc2). Each fusion emits a tiny bit of energy and a gazillion fusion reactions take place per second and the stars give off this energy as light and heat.

Nuclear fusion process in the sun
Figure 03: Nuclear fusion process in the sun

Even if brown dwarfs are not stars, some of them are able to retain nuclear fusion for a short period of time. They use deuterium (an isotope of hydrogen) for the fusion and once the deuterium cache is all used up, they begin to contract, cool off, and their brightness in different wavelengths of light changes. As they glow rather brightly emitting infrared light, telescopes like Spitzer and Wide-field Infrared Survey Explorer (WISE) can pick up and map brown dwarfs. Even with infrared telescopes, it is sometimes hard to discover brown dwarfs as they are elusive due to their inherent faintness and relatively low temperature.

The Unorthodox Brown Dwarf that Defied all Expectations

The Accident defied all expectations as it seemed faint in some infrared wavelengths inferring it was a cold and old brown dwarf, but also it appeared bright in other wavelengths, implying that it was a warm, young brown dwarf. Due to these puzzling contradictions, The Accident was observed in additional infrared wavelengths using a ground-based telescope at the W. M. Keck Observatory in Hawaii. With extra data, it was confirmed that The Accident happens to be very cold as it appeared faint in those additional infrared wavelengths.

Due to its dimness, the scientists assumed that The Accident should be farther than expected from the Earth, but it happens to be located about 50 light years away from Earth. The astronomers were baffled by its more puzzling characteristics as it was moving so fast at about 800,000 km/h, more quickly than a typical brown dwarf at this distance from Earth. Therefore, it was deduced that The Accident is very old, and its accelerating movement is due to jostling around for billion years by the gravity of larger objects.

The atmosphere of typical brown dwarfs with temperatures similar to The Accident contains methane. Yet, the astronomers found out based on the wavelengths of infrared emitted by the object, that it is low in methane. The scarcity of methane suggests that The Accident might be very old; 10 billion to 13 billion years old when the Milky Way was much younger filled with Hydrogen and Helium and less Carbon.

All this evidence implies that The Accident is atypical being very cold and exceptionally old.

An illustration depicting a dim, cold brown dwarf in space.
Figure 04: An illustration depicting a dim, cold brown dwarf in space. Credits: NASA/ JPL-Caltech

It’s not a surprise to find a brown dwarf this old, but it is a surprise to find one in our backyard… We expected that brown dwarfs that this old exist, but we also expected them to be exceptionally rare. The chance of finding one so close to the solar system could be a lucky coincidence, or it tells us that they’re more common that we thought.

Federico Marocco, an astrophysicist, IPAC at Caltech

The universe still is not fully unfolded and there is plenty to be unveiled. With the knowledge gathered and analyzed, the astronomers are now looking for similar objects like The Accident in the universe and want the next ‘The Accident’ not to be an accident.

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