Bewitching, bewildering brown dwarfs are genuine “oddballs”–and their existence in the Universe presents astronomers with an intriguing mystery to solve. This is because these stellar “failures” challenge the neat distinction between true stars and giant planets. Brown dwarfs are substellar objects that occupy a mass-range between the heaviest gas-giant planets–such as our own Solar System’s banded behemoth Jupiter–and the lightest of true stars, which are red dwarfs. Unlike true stars–like our Sun–these tiny, tiny brown dwarfs are not massive enough to sustain nuclear fusion, which is the process that lights a true star’s fires–and causes them to send their light shining through the Universe. In January 2018, astronomers said that they are hopeful that NASA’s upcoming James Webb Space Telescope’s (JWST’s) powerful infrared capacity will resolve a puzzle as mesmerizing as age-old observations of starlight–when, thousands of years ago, our ancestors gazed upward on dark, starry nights, and wondered what could be the source of a faint, faraway little dab of mysterious light glimmering like a diamond in the night sky above Earth?
A number of scientific research teams plan to use the JWST to explore the mysterious nature of brown dwarfs, seeking new insight into both star formation and the nature of atmospheres sported by distant exoplanets in orbit around stars beyond our Sun. Several teams of astronomers also plan to investigate the murky territory, in-between true stars and giant planets, where brown dwarfs themselves exist. Earlier work performed by astronomers using the Hubble Space Telescope, Spitzer Space Telescope, and ALMA have demonstrated that brown dwarfs can be as much as 70 times more massive than gas-giants like Jupiter–and yet they do not have sufficient mass for their cores to burn nuclear fuel and, thus, radiate starlight. Even though the existence of these stellar “failures” was first proposed back in the 1960s, and confirmed in 1995, there is still no accepted explanation for how they form. Are brown dwarfs born like stars, by the contraction of gas within a star-birthing cloud, or like a planet by the accretion of material within a protoplanetary accretion disk? Some “failed”stars live in close proximity to a companion star, while others wander their solitary way through space.
One particularly nagging problem is that gas-giants display some of the same characteristics as brown dwarfs. This, of course, makes it difficult for astronomers to distinguish between a “failed” star and a gas-giant planet. Brown dwarfs can sometimes be observed in orbit around true stars or other brown dwarfs, and the tiniest brown dwarfs show sizes that are similar to that of Jupiter. This means that these little “oddballs” would look eactly like a planet in orbit around its parent-star.
Stellar “Failures”: An Iron Rain’s A Gonna Fall
Brown dwarfs are substellar “oddballs”, that occupy the mass range intermediate between the heaviest gas-giant planets and the lightest red dwarfs, of approximately 13 to 80 Jupiter-masses. These tiny substellar objects, that are not really brown, but purple-pink in color (magenta), may be fully convective, possessing no layers or chemical differentiation with depth.
Most astronomers think that brown dwarfs are born just like true stars, within the billowing, swirling folds of one of the many giant, dark, and frigid molecular clouds that haunt our Milky Way Galaxy in huge numbers. These huge, beautiful, ghostly clouds, composed of gas and dust, sparkle with the newborn stellar flames of baby stars. Stars are born when an especially dense pocket, embedded within the whirling swirls of such a cloud, collapses under the merciless squeeze of its own gravity–forming a newborn star.
Sparkling, dazzling newborn stars–termed protostars–are cradled within the contracting blob of gas within its natal cloud. At the time of star-birth, the temperature at the center of the dense blob rises to the searing-hot point that hydrogen atoms start to fuse together to form helium atoms (stellar nucleosynthesis). Hydrogen is both the lightest and most abundant atomic element in the Cosmos–and helium is the second-lightest. All stars are composed primarily of hydrogen, and both hydrogen and helium were formed in the Big Bang birth of the Universe almost 14 billion years ago (Big Bang nucleosynthesis).
But, the existence of gas-giant planets complicate the issue. At the high end of the mass-range–60 to 90 Jupiter-masses–the volume of a brown dwarf is controlled mainly by electron-degeneracy pressure. This is the same as it is for white dwarfs, which are the remnant cores left behind by small true stars, like our own Sun, after they have used up their necessary supply of fuel, and have “gone gentle into that good night”. At the low end of the mass-range–10 Jupiter-masses–the volume of a brown dwarf is controlled the same way that it is for planets. Hence, the problem: the radii of brown dwarfs vary by a mere 10-15% over the range of possible masses for giant-planets. This makes it difficult for astronomers to distinguish between the two different objects.
In addition, brown dwarfs never experience nuclear fusion. Those stellar “failures” that occupy the low end of the mass-range (under 13 Jupiter-masses) never become hot enough to even fuse deuterium. Complicating things even further, those “failed” stars that occupy the high-end of the mass-range for their tiny kind (more than 60 Jupiter-masses) cool off so rapidly that, after about ten million years they can no longer undergo nuclear fusion. Ten million years is not a long time for a star.
Infrared and X-ray spectra are tattle-tale signs of a brown dwarf. Some of these “failed” stars emit X-rays and all “warm” brown dwarfs continue to gently glow in the red and infrared electromagnetic spectra. For this reason, these stellar “failures” give themselves away to astronomers–at least until they cool off to more planet-like temperatures of under 1000 Kelvin.
Some gas-giant planets show several of the characteristics of brown dwarfs. Like our Sun ( a yellow dwarf), our Solar System’s gas-giant duo, Jupiter and Saturn, are both mainly composed of hydrogen and helium. Saturn is the second-largest planet in our Sun’s family, despite containing only about 30% of the mass of its larger sibling world, Jupiter. Three of the giant gaseous planets in our Solar System–Jupiter, Saturn, and Uranus–emit much more heat than they receive from our Sun. The entire quartet of giant gaseous planets in our Sun’s family–Jupiter, Saturn, Uranus, and Neptune–have “planetary systems” of their own, which are composed of many–mostly-icy–moons. Two large moons–Ganymede of Jupiter and Titan of Saturn–are planet-sized worlds that would be categorized as planets if they orbited our Sun instead of Jupiter and Saturn. Ganymede is the largest moon in our Solar System, and Titan is the second-largest. Although Uranus and Neptune are giant planets with heavy gaseous atmospheres, they are both categorized as ice-giants rather than gas-giants. Uranus and Neptune are smaller than the gas-giant duo, and both possess larger solid cores enshrouded beneath their atmospheres.
Brown dwarfs were first theorized to exist back in the 1960s by Dr. Shiv S. Kumar, and they were originally dubbed “black dwarfs”–a classification for dark substellar bodies, floating freely through interstellar space, that are too light to sustain hydrogen fusion. However, this name could not be used because the term black dwarf was already being used to designate a cold white dwarf, that had faded to black, after all of its heat had dissipated.
At a distance of about 6.5 light-years, the closest known brown dwarf is Luhman 16, a binary system composed of a duo of these stellar “failures”, discovered in 2013. DENIS-P Jo82303.1-49120 b is listed as the most-massive known exoplanet (as of March 2014) in NASA’s exoplanet archive–even though it sports an impressive mass that is more than double that of the 13-Jupiter-mass cutoff between giant planets and brown dwarfs. Some brown dwarfs are substellar-parents to planets. A few examples of these substellar parent-stars are 2M1207b, MOA-2007-BLG-102Lb, and 2MASS Jo44144b.
Lithium is usually present in brown dwarfs, but not in low-mass true stars. Stars, which manage to reach a temperature roasting enough to fuse hydrogen, very quickly burn their lithium fuel. Fusion of lithium-7 and a proton occurs manufacturing two helium-4 nuclei. The temperature that is necessary for this reaction to occur is a little below that necessary for hydrogen fusion. Convection within a star of low mass ensures that lithium contained by the entire star is ultimately used up as fuel. As a result, the presence of the spectral line of lithium in a brown dwarf candidate is a strong indicator that it is indeed one of these substellar runts.
The use of lithium to determine whether a candidate brown dwarf is indeed a stellar “failure”, and not a true star of low mass, is commonly referred to as the lithium test. Lithium is also observed in young stars, which have not yet had sufficient time to burn all of it.
Heavier stars, like our Sun, can also hold on to their supply of lithium in their outer gaseous layers, which never reached a temperature sufficiently hot to fuse lithium–and whose convective layer does not combine with the core where the lithium would be quickly used up. Those larger stars are readily distinguishable from brown dwarfs by their larger size and brighter luminosity.
However, brown dwarfs on the high end of the mass range can be sufficiently hot to burn up their lithium when they are still youngsters. Dwarfs that sport a mass greater than 65 Jupiter-masses can burn up their supply of lithium by the time they are “only” half a billion years old. For these reasons, the lithium test is not completely reliable.
Older brown dwarfs, on the other hand, are sometimes cool enough for their atmospheres to collect observable amounts of methane over long spans of time. Methane is is unable to form in hotter objects. Stellar “failures”, such as Gliese 229B, have been verified by astronomers using this particular technique.
Searing-hot stars, that are still on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution, eventually cool off. Ultimately, these stars reach a bolometric liminosity that that is sufficient for them to sustain a steady fusion. However, one star is not exactly like another, and so this varies from star to star. However, the bolometric luminosity must generally be at least 0.01% that of our Sun. Brown dwarfs grow dark and cool off at a steady rate over the course of their lifetimes: sufficiently elderly brown dwarfs are too dim to be observed.
Iron rain as part of atmospheric convection processes can only occur in brown dwarfs, and not in small stars such as red dwarfs. The spectroscopy research into the presence of iron rain is currently ongoing, but not all brown dwarfs will always show this bizarre atmospheric anomaly. In 2013, a heterogeneous iron-containing atmosphere was imaged around the stellar B component in the closely knit Luhman 16 binary stellar system.
Tiny, Tiny Failed Star: How We Wonder What You Are!
University of Montreal (Quebec, Canada) professor Etienne Artigau leads a team that plans to use the JWST to study a specific brown dwarf dubbed SIMP0136. This stellar “failure” is a low-mass, solitary, young brown dwarf that has the distinction of being one of the closest of its tiny kind to our Sun. All of these bewitching features make SIMP0136 an intriguing object for study. This is because it possesses numerous features of a giant planet that is not too close to the blinding glare of its parent-star. SIMP0136 was the object of an earlier scientific breakthrough made by Dr. Artigau and his team, when they discovered clues indicating that it sports a cloudy atmosphere. He and his colleagues plan to use JWST’s spectroscopic instruments to discover more about the chemical elements and compounds that exist within those clouds.
“Very accurate spectroscopic measurements are challenging to obtain from the ground in the infrared due to variable absorption in our own atmosphere, hence the need for space-based infrared observation. Also, Webb allows us to probe features, such as water absorption, that are inaccessible from the ground at this level of precision,” Dr. Artigau explained in a January 4, 2018 NASA Press Release.
These observations could lay groundwork for exoplanet exploration, using JWST, when it is available. These future observations include the hunt for distant worlds, belonging to the families of stars beyond our Sun, that could support life. JWST’s infrared instruments will have the capability of spotting the types of molecules in the atmospheres of faraway exoplanets. These instruments will do this by observing which elements are absorbing light as the distant world floats in front of the glaring face of its star–in a scientific technique termed transit spectroscopy.
“The brown dwarf SIMP0136 has the same temperature as various planets that will be observed in transit spectroscopy with Webb, and clouds are known to affect this type of measurement; our observations will help us better understand cloud decks in brown dwarfs and planet atmospheres in general,” Dr. Artigau continued to explain in the NASA Press Release.
The hunt for solitary, isolated low-mass brown dwarfs was one of the first science goals proposed for the JWST back in the 1990s, Dr. Aleks Scholz said in the January 4, 2018 NASA Press Release. Dr. Scholz is an astronomer of the University of St. Andrews in Scotland. Brown dwarfs have a lower mass than stars and do not “shine”, but merely give off the faint afterglow of their birth. For this reason, brown dwarfs can best be observed in infrared light, which is why the JWST will be such an important tool to be used in this research.
Dr. Scholz, who also leads the Substellar Objects in Nearby Young Clusters (SONYC) project, will use JWST’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) in order to study NGC 1333 located in the constellation of Perseus. NGC 1333 serves as a nursery for fiery stellar newborns. In addition, it has also been found to host an unusually high number of brown dwarfs, and some of them are at the very low end of the mass range for these puzzling objects. This means that these bewitching brown dwarfs are not much heavier than Jupiter.
“In more than a decade of searching, our team has found it very difficult to locate brown dwarfs that are less than five Jupiter-masses–the mass where star and planet formation overlap. That is a job for the Webb telescope. It has been a long wait for Webb, but we are very excited to get an opportunity to break new ground and potentially discover an entirely new type of planets, unbound, roaming the Galaxy like stars,” Dr. Scholz commented in the January 4, 2018 NASA Press Release.
Both of the projects led by Dr. Scholz and Dr. Artigau are making use of Guaranteed Time Observations (GTOs), observing time on the telescope that is granted to astronomers who have worked for years to prepare Webb’s scientific operations.
The James Webb Space Telescope, the scientific complement to NASA’s very successful Hubble Space Telescope (HST), will become the premier space observatory for the next decade. Webb is an international project led by NASA with its partners, the European Space Agency (ESA) and Canadian Space Agency (CSA).
The James Webb Space Telescope is scheduled to launch in 2019.