An accidental discovery hints at a hidden population of cosmic objects

This mosaic depicts the entire sky as seen by Wide-field Infrared Survey Explorer. Infrared light is a wavelength that is longer than what the human eye can see. Infrared light is radiated by many cosmic objects, including dust clouds and gas clouds that form stars, as well as brown dwarfs. Credit: NASA/JPL-Caltech/UCLA
Brown dwarfs are not stars or planets. A new study suggests that there may be more brown dwarfs in the galaxy than previously thought.

A new study offers a tantalizing explanation for how a peculiar cosmic object called WISEA J153429.75-104303.3nicknamed "The Accident"came to be. The Accident is a brown dwarf. These objects form like stars but don't have enough mass for nuclear fusion to ignite the process that makes stars shine. Brown dwarfs can be difficult to describe, but astronomers are able to identify their basic characteristics.

They did until they found one.

After being found by pure luck, the Accident was named. Because it isn't like any of the nearly 2,000 brown dwarfs found in our galaxy, normal searches failed to find it.

Brown dwarfs get darker as they age and their brightness changes with different wavelengths of light. This is similar to how certain metals change from being brightly white to deep red when they cool down after being heated. Scientists were confused by the Accident because it was very faint in certain wavelengths. This indicated that it was very old and cold, but it was brighter in others, which indicates a higher temperature.

Davy Kirkpatrick (an astrophysicist at Caltech Pasadena) said that "this object defied all of our expectations." In their latest study, published in The Astrophysical Journal Letters they suggest that The Accident could be between 10 and 13 billion years old, which is at least twice the age of other brown dwarfs. This would mean that it formed in a much younger galaxy with a different chemical composition. This would explain why there are many more ancient brown dwarfs in our galaxy.

A peculiar profile

Brown dwarfs have certain similarities to stars and planets. They are generally smaller than stars, and larger than planets. If the core pressure of a brown dwarf is high enough to trigger nuclear fusion, which causes stars to shine, it becomes a star. Credit: NASA/JPL-Caltech

NASA's Near-Earth Object Wide-Field Infrared Survey Explorer, (NEOWISE) first spotted the Accident in 2009. Launched in 2009 under WISE, it is managed by NASA's Jet Propulsion Laboratory. Brown dwarfs, which are cool objects, emit mostly infrared radiation, or wavelengths that are longer than the human eye can see.

Scientists needed more information to understand how The Accident could possess seemingly contradictory properties. Some suggest it is very cold while others indicate it is much warmer. They observed it in infrared wavelengths using a ground-based telescope from Hawaii's W. M. Keck Observatory. The brown dwarf was so faint that they couldn't see it in these wavelengths. This seems to confirm their suspicions that it was very cold.

Next, they set out to find out if the dimming was due to The Accident being further than expected from Earth. NASA's Spitzer Space Telescopes and Hubble Space Telescopes proved otherwise. The team discovered that the object is traveling at a speed of half a million miles an hour (880,000 kph) after determining its distance from Earth. This is faster than any other brown dwarfs at this distance from Earth. It means that it has been traveling around the galaxy for some time and may have encountered massive objects that accelerate it.

The researchers have gathered a lot of evidence that suggests that The Accident is very old. They suggest that its bizarre properties aren’t unusual and may be a clue as to its age.

The Milky Way was almost entirely hydrogen and helium when it formed around 13.6 billion years ago. Carbon and other elements formed in stars. The most massive stars that exploded as supernovae scattered these elements across the galaxy.

The majority of brown dwarfs with a similar temperature to The Accident contain methane. It is composed of carbon and hydrogen. The Accident's light profile indicates that it has very little methane. Methane, like all molecules absorbs certain wavelengths of light. A methane-rich brown dwarf would therefore appear dim in these wavelengths. However, the Accident is brighter in these wavelengths which could indicate low levels methane.

The Accident's light profile could be compared to that of a brown dwarf formed in a galaxy that was still carbon poor. Very little carbon at its formation equals very little methane today.

The illustration of a cold, dim brown dwarf in space is by this artist. Brown dwarfs are like stars but lack enough mass to ignite nuclear fission in their cores, which is what causes stars to explode. They share some physical characteristics and features with large planets like Jupiter. Credit: IPAC/Caltech

Federico Marocco, an IPAC at Caltech astrophysicist, said, "It's not surprising to find a brown dwarf that old, but it's a surprise for one to be found in our backyard." Federico Marocco was the lead of the new observations made with the Keck- and Hubble telescopes. We expected brown dwarfs of this age to exist but they are rare. It could be luck, or they may be more common than we thought.

A lucky accident

Researchers might need to modify their search methods in order to find more brown dwarfs such as The Accident if they are out there.

Citizen scientist Dan Caselden discovered the Accident while using an online program he had built to search for brown dwarfs in NEOWISE data. There are many objects in the sky that emit infrared light. Due to their distance from Earth, they appear to be fixed in space. Brown dwarfs are very faint and can only be seen when they are relatively close to Earth. Scientists can then observe their movement across the sky for months or even years. (NEOWISE maps approximately the entire sky every six months.

Caselden's program tried to eliminate stationary infrared objects (like distant star) from the NEOWISE charts and highlight moving objects with similar characteristics to brown dwarfs. As he was looking at one candidate for brown dwarf, he noticed another object that moved quickly across the screen. It would be WISEA J153429.75-104303.3. This object was not highlighted as it didn't match the program's description of a brown dwarf. Caselden was lucky to have it.

Kirkpatrick stated, "This discovery is tellings us that there are more variety in brown dwarf compositions then we've seen so far." "There's likely to be more bizarre ones out there and we need to learn how to find them."

The WISE spacecraft, which was launched in 2009, was put into hibernation in 2011. It had completed its primary mission. NASA reactivated WISE in September 2013 with the primary purpose of scanning for near Earth objects (NEOs) and renamed the spacecraft and mission NEOWISE. JPL, a division at Caltech, managed WISE for NASA's Science Mission Directorate. NASA's Explorers Program, managed by Greenbelt's Goddard Space Flight Center in Greenbelt (Maryland), selected the mission competitively. NEOWISE, a NASA-supported project, is a joint venture of JPL and Caltech's University of Arizona. It was funded by NASA's Planetary Defense Coordination Office.

Continue reading Unraveling brown dwarf mysteries

Additional information: J. Davy Kirkpatrick, The Enigmatic Brown Dwarf WISEA J153429.75-104303.3 (a.k.a. "The Accident", The Astrophysical Journal Letters (2021). Journal information: Astrophysical journal Letters J. Davy Kirkpatrick, The Enigmatic Brown Dwarf WiSEA J153429.75-104303.3 (a.k.a. "The Accident "),(2021. DOI: 10.3847/2041-8213/ac0437