Brown Dwarfs – are they planets, stars, or something entirely different?

In November 2018, amateur astronomer Dan Caselden was running an automated search of NASA space telescope images on his computer, for the Backyard Worlds: Planet 9 project, when he found something weird. About 50-light-years away from Earth, in a field imaged by the Wide-field Infrared Survey Explorer, this object was extremely faint and traveling at 200 km per second. They named it WISE 1534-1043 and also The Accident, due to the way it was discovered.

What are brown dwarfs?

What was The Accident? Astronomers think it was a brown dwarf. And what are brown dwarfs? They are substellar objects that do not have enough mass to fuse hydrogen in their cores, like our Sun does and so do other stars. So, they are not stars, although they begin their lives like stars but then fail to reach their full potential and do not start nuclear fusion in their cores. But they are also not planets. They lie between planets and stars.

Brown dwarfs have a mass between the most massive gas giants in our solar system – like Jupiter  – and the least massive stars. They lie between 13 to about 80 times the mass of Jupiter. However, scientists do not know where exactly those boundaries lie. At 13 Jupiter masses, brown dwarfs are able to fuse deuterium (a rare isotope of hydrogen) and those greater than 65 Jupiter masses can fuse lithium. This deuterium fusion is how stars produce energy in the first few million years of their lives. Stars then continue to contract and get hotter till they start fusing hydrogen. Brown dwarfs on the other hand do not contract further and most of them do not start fusing hydrogen. Instead, their cores are dense enough to hold themselves up due to something called electron degeneracy pressure i.e the pressure exerted by densely packed electrons that differ from each other in terms of momentum. Brown dwarfs above 60 Jupiter masses do burn hydrogen for a bit, but then they stabilize and the fusion stops.

Astronomical objects that are self-luminescent (that is those that emit their own light) are classed according spectral classes, a distinction closely tied to surface temperatures. Based on this, brown dwarfs are classified as types M, L, T and Y.

They are called brown dwarfs (a name proposed by astronomer Jill Tarter in 1975) but they come in a variety of colours depending on their temperatures. The warmest ones are likely orange or red in colour, while cooler brown dwarfs would probably appear magenta to the human eye. Their temperatures range from 2,000 degrees Celsius to below 200 Celsius. This lower range is what they cool towards over billions of years, as they do not have their own inner heat source. Brown dwarfs may have atmospheres and large cloud masses, so it is thought that turbulent storms might be common on them. But because these clouds are too hot to be composed of water, it is also thought that they are made up of sand and molten iron. So, brown dwarfs can also have iron rain. They also have low surface gravities due to large radii and low mass. In the year 2020, wind speeds of a brown dwarf 34 light-years away were measured. This was calculated to be over 2,300 km per hour — more than five times that of Jupiter’s winds (Jupiter, Solar System’s Giant)

These strange objects were first theorized in the 1960s by Shiv S. Kumar and were originally called black dwarfs, but this term was already used to refer to cold white dwarfs. It was not until the 1990s that the first brown dwarfs were discovered. Brown dwarfs have low surface temperatures and are not very bright at visible wavelengths, emitting electromagnetic radiation in infrared. It was only after our infrared detecting instruments got more advanced were we able to identify them. And since then thousands have been found.

Types of brown dwarfs

The nearest-known brown dwarfs are in the Luhman 16 system, a binary system of L- and T-type brown dwarfs about 6.5 light-years away from us. Luhman 16 is the third-closest system to the Sun after Alpha Centauri and Barnard’s Star (which is a red dwarf star). In 1988, a faint companion to a star named GD 165 was found in an infrared search of white dwarfs. This companion, termed GD 165B, was red and did not show any of features associated with a low-mass red dwarf. Today, GD 165B is acknowledged as a prototype of a L class brown dwarf or L dwarf.

The first class T brown dwarf was discovered in 1994, which was confirmed as a substellar companion to Gliese 229. This was the first ever clear evidence of a brown dwarf, along with another one called Teide 1. Teide 1 established the M class in 1994 and was classed as such due to its methane absorption – a feature not observed in a main sequence star.

The coolest brown dwarfs fall into category Y, with temperatures below 227–327 degrees Celsius or 440–620 degrees Fahrenheit. On 25 April 2014, the oldest-known brown dwarf was discovered, known as WISE 0855−0714. It is 7.2 light-years away (seventh-closest system to the Sun) and has a temperature of between −48 to −13 degree Celsius.

Around 50 Y dwarfs have been found. The one I mentioned before, The Accident, is a lone brown dwarf. And it is a strange one. Its absolute brightness at different wavelengths is in line with the coldest known Y dwarfs. But its relative colours and magnitudes are entirely outside of the range of known brown dwarfs. Among a few reasons for this anomaly scientists think the most likely is because it is an old, cold brown dwarf, with low metallicity. That is, it is metal poor, so it was probably created before the Milky Way galaxy had all the metals it does now. It was probably one of the first brown dwarfs formed in our galaxy. It may very well be a brand-new category of star – the first known Y-type subdwarf. Obviously more observations are needed to confirm this.

Fastest brown dwarfs

Using data from NASA’s Spitzer Space Telescope, scientists have also identified the three fastest-spinning brown dwarfs ever found. In a study, the team that made the new speed measurements suggest that these three high speed rotators could be approaching a spin speed limit for all brown dwarfs, beyond which they would break apart. All three are about the same diameter as Jupiter but are between 40 to 70 times more massive. They each rotate about once per hour, while the next-fastest known brown dwarfs rotate about once every 1.4 hours and Jupiter spins once every 10 hours. Based on their size, that means the largest of the three brown dwarfs goes around at more than 100 km per second, or about 360,000 km per hour. Brown dwarfs, like stars or planets, are already spinning when they form. As they cool down and contract, they spin faster. Scientists have measured the spin rates of about 80 brown dwarfs, which vary from less than two hours (including the three new entries) to tens of hours (Saturn: How Cassini Made the Earth Smile)

Because these objects lie between planets and stars, they can help us understand both, and also how nuclear fusion begins in stars. Astronomers think that an object needs to reach temperatures of 3 million degrees Celsius at its core for nuclear fusion to begin but they do not know what mass is needed for this to happen. Scientists recently identified five high-mass brown dwarfs with masses between 77 and 98 times that of Jupiter – the boundary of where hydrogen starts to fuse into helium. But it is still unclear which side of the boundary they are at. Some brown dwarfs are so similar to stars they could potentially host their own planets. However, none have been found with an orbiting exoplanet as yet.

(Pluto, The Planet that wasn’t)

Research continues to help scientists – and us – understand these strange objects in our skies.

All images: NASA/ESA/Hubble/JPL-Caltech

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Saima Baig

I am a Chartered Environmentalist from the Royal Society for the Environment, UK and co-owner of DoLocal Digital Marketing Agency Ltd, with a Master of Environmental Management from Yale University, an MBA in Finance, and a Bachelor of Science in Physics and Mathematics. I am passionate about science, history and environment and love to create content on these topics.