A Giant Planet Or A Tiny Stellar Failure?



A Giant Planet Or A Tiny Stellar Failure?
By Judith E Braffman-Miller

Brown dwarfs are clearly the runts of the stellar litter, and are often referred to as “failed stars” because their meager masses are unable to light their stellar fires by way of the process of nuclear fusion–and, for this reason, brown dwarfs cannot shine like true stars. “Failed stars” are born like their more massive, and more successful, sparkling stellar kin–from the collapse of a particularly dense blob tucked within the swirling, whirling folds of a cold, giant, dark, molecular cloud–but, tragically, they are doomed to never grow up. Brown dwarfs are approximately the same radius as our own Solar System’s banded behemoth, gas-giant, Jupiter. The problem is that gas-giants possess some of the characteristics of brown dwarfs–thus making it difficult for astronomers to distinguish between a gas-giant planet and a “failed star”. In December 2017, a team of astronomers from the California Institute of Technology (Caltech) in Pasadena, announced that they have taken a new approach to solving this stellar mystery. The researchers have compared the spin rates of brown dwarfs to gas-giant planets, and their results offer a new set of clues that hint at how these two distinct celestial objects are born.

It is not easy for an astronomer to obtain an image of an exoplanet, in orbit around a distant star beyond our own Sun. This is because the brilliant light of a planet’s parent-star overwhelms the comparatively weak light emanating from its circling planet. This makes the planet difficult to detect. However, even though taking a picture of a rocky, small planet like our own is still not possible using current technology, astronomers have managed to successfully snap pictures of about 20 giant planet-like objects belonging to the remote families of stars other than our own. These “oddball” objects, collectively termed planetary-mass companions, are more massive than Jupiter, and orbit far from the overwhelming glare of their parent-stars. These mysterious planetary-mass companions are still youthful enough to softly shine with the heat lingering from their formation. All of these traits make these distant objects easier for astronomers to image.

However, one nagging question remains unanswered. Are these planetary-mass companions really planets, or are they the stellar failures that we call brown dwarfs? Brown dwarfs are born like other stars–but they lack the mass to shine with starlight. These distant objects can be observed freely floating through interstellar space with no family of their own, or they can be seen circling other brown dwarfs or true stars. The smallest brown dwarfs sport sizes comparable to that of Jupiter–and they would look exactly like a planet in orbit around its star.

Astronomers at Caltech have taken a new approach to solving this mystery. The scientists have measured the spin rate of a trio of the photographed planetary-mass companions and compared them to the spin rates observed for small brown dwarfs. Their findings provide a new set of clues about how these mysterious companions may have been born.

“These companions with their high masses and wide separations could have formed either as planets or brown dwarfs. In this study, we wanted to shed light on their origins,” commented graduate student Marta Bryan in a December 4, 2017 Caltech Press Release. Bryan is the lead author of the new study describing findings published in the journal Nature Astronomy.

“These new spin measurements suggest that if these bodies are massive planets located far away from their stars, they have properties that are very similar to those of the smallest brown dwarfs,” explained Dr. Heather Knutson in the same Caltech Press Release. Dr. Knutson is a professor of planetary science at Caltech and a co-author of the study.

Bewitching, Bewildering Objects Inhabiting The Space Between Stars

Brown dwarfs are true oddities. They are both bewitching and bewildering, and their existence presents a puzzle because they challenge the neat distinction between true, nuclear-fusing stars and giant planets. Many astronomers think that these relatively petite and cool inhabitants of the Universe are born just like their true stellar kin, within the ghostly, undulating billows of one of the many gigantic, dark, and cold molecular clouds that haunt interstellar space like lovely, mysterious, floating phantoms, sparkling with the newborn fires of baby stars. Stars are born when a particularly dense blob embedded within the folds of one of these clouds collapses under the relentless pull of its own gravity–thus giving birth to the new star.

Brilliant, dazzling baby stars–called protostars–are cradled within the contracting, especially dense cloud of gas. At the time of stellar birth, the temperature at the heart of the dense blob skyrockets to the sizzling point that hydrogen atoms begin to fuse together to create helium atoms. Hydrogen is both the lightest and most abundant atomic element in the Cosmos, and helium is the second-lightest. All stars are mainly made up of hydrogen, and both hydrogen and helium were formed in the fireball of the Big Bang that occurred at the moment of our Universe’s birth almost 14 billion years ago.

However, gas-giant planets confuse the issue. At the high end of the mass-range–60 to 90 Jupiter-masses–the volume of a brown dwarf is controlled primarily by electron-degeneracy pressure–just as it is for white dwarfs, which are the relic cores left behind by small stars, like our Sun, after they have perished. At the low end of the mass-range–10 Jupiter-masses–the volume of a brown dwarf is controlled exactly the same way that it is for planets. The problem is this–the radii of brown dwarfs vary by a mere 10-15% over the range of possible masses for their giant-planet-distant-cousins. This makes the task of distinguishing brown dwarfs from giant planets very difficult.

Furthermore, many brown dwarfs never undergo the process of nuclear fusion. Those “failed stars” that occupy the low end of the mass-range (under 13 Jupiter-masses) never grow hot enough to even fuse deuterium–and those brown dwarfs that occupy the high end of the mass range for their puny kind (more than 60 Jupiter-masses) cool off so quickly that, after about 10 million years (a mere blink of the eye in the “life” of a stellar failure), they can no longer undergo nuclear fusion.

Infrared and X-ray spectra are tattle-tale signs of brown dwarfs. Some brown dwarfs emit X-rays and literally all “warm” brown dwarfs continue to glow gently in the red and infrared spectra. In this way, these “failed stars” reveal their identity to curious astronomers–that is, until they cool off to more planet-like temperatures (under about 1000 Kelvin).

The problem is, of course, that some gas-giant planets possess several of the characteristics of their brown dwarf distant cousins. Like our own Sun, Jupiter and Saturn (the smaller of our Solar System’s gas-giant duo), are both primarily composed of hydrogen and helium. Saturn is almost as large as Jupiter, despite possessing only 30% the mass. A trio of the giant gaseous planets in our Sun’s family–Jupiter, Saturn, and Uranus–emit considerably more heat than they receive from our Star. The entire quartet of gaseous giant planets in our Solar System–Jupiter, Saturn, Uranus, and Neptune–have “planetary systems” of their own, which are made up of their moons. Like their stellar kin, brown dwarfs are born independently. But, alas, unlike their more successful larger cousins, they never manage to gain sufficient mass to ignite, and burn brightly with furious stellar fires. Brown dwarfs, like true stars, can be born singly or in close proximity to other stars. Some brown dwarfs orbit other stars, and can–like planets–sport eccentric orbits.

The International Astronomical Union (IAU), currently, defines an object above 13 Jupiter-masses–which is the limiting mass for thermonuclear fusion of deuterium–to be a brown dwarf. In contrast, the IAU classifies an object under that mass (and in orbit around a star or stellar remnant) as a planet.

The 13 Jupiter-mass cut-off does not really have any precise physical significance. Larger stellar objects will burn most of their supply of deuterium, while smaller ones will burn only a little–and the 13 Jupiter-mass value occupies a middle ground. The quantity of deuterium burnt by the stellar object also depends to some extent on its composition–specifically on the amount of helium and deuterium present and on the percentage of heavier atomic elements, which determines the atmospheric opacity and thus the radiative cooling rate of the object.

The Extrasolar Planets Encyclopedia includes objects up to 25 Jupiter-masses, and the Exoplanet Data Explorer up to 24 Jupiter-masses.

When a star is born within its natal cloud composed of gas and dust, the disk of orbiting material that encircles it–the protoplanetary accretion disk–can ultimately form a family of orbiting planets. Therefore, planets form from the material in the primordial dusty and gaseous disk that circles around a young star. In the first stages of star-birth, jets of material stream outward from the poles. However, no such jets are propelled outward to herald the birth of a planet.

A Giant Planet Or A Tiny Stellar Failure?

The Caltech astronomers used the W.M. Keck Observatory in Hawaii–which is managed by Caltech, the University of California, and NASA–to measure the length of a day (the spin rate), of a trio of planetary mass companions named ROXs 42B b, GSC 6214-210 b, and VHS 1256-1257 b. The scientists used an instrument at Keck called the Near Infrared Spectrograph (NIRSpec) to study the light emanating from the companions. As the objects spin on their axes, light flowing out from the side that is turning toward us shifts to shorter, bluer wavelengths (blueshift), while the light from the receding side shifts to longer, redder wavelengths (redshift). The amount of shifting indicates how fast the object is spinning. The results showed that a trio of companions spin rates ranged between 6 and 14 kilometers per second. This is similar to the rotation rates of our own Solar System’s planetary behemoths, Jupiter and Saturn.

For their research, the scientists compared the spin rates for the five companions that had previously been measured, looking for small free-floating brown dwarfs. The rotation rate ranges for the two populations proved to be indistinguishable from one another. This means that the companions are twirling around on their axes at approximately the same speeds as their free-floating brown dwarf kin.

The new results offer two possible explanations. The first is that the planetary-mass companions observed during this study are planets that were born, just like planets normally do, out of whirling, swirling protoplanetary accretion disks of gas and dust, circling their young parent-stars. However, for reasons not yet understood, the objects wound up with spin rates similar to those sported by brown dwarfs. Some astronomers propose that both neonatal planets and brown dwarfs are surrounded by miniature disks of gas that could be helping to slow down their spin rates. This indicates that similar processes may leave both giant planets and brown dwarfs with similar spin rates.

“It’s a question of nature versus nurture. Were the planetary companions born like brown dwarfs, or did they just end up behaving like them with similar spins,” noted Dr. Knutson in the December 4, 2017 Caltech Press Release.

The astronomers also note that the companions are spinning more slowly than they had predicted. Growing young planets tend to be spun up by the material they sip in from the surrounding disk of gas. This has been compared to the way that a spinning ballerina increases her speed, or angular momentum, when she pulls her arms in. The relatively lazy rotation rates observed for these baffling objects suggest that they were able to successfully slow themselves down during this spin-up process–possibly by transferring some of this angular momentum back to surrounding disks of gas. The scientists are planning future studies of spin rates to further understand the matter.

The new study is titled Constraints on the Spin Evolution of Young Planetary-Mass Companions. Other authors include Caltech’s Dr. Konstantin Batygin, assistant professor of planetary science and Van Nuys Page Scholar, Dr. Bjorn Benneke, also of Caltech.

Marta Bryan noted in the December 4, 2017 Caltech Press Release that “Spin rates of planetary-mass bodies outside our Solar System have not been fully explored. We are just now beginning to use this as a tool for understanding formation histories of planetary-mass objects.”

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.

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