What makes a planet? Daniela Bardalez Gagliuffi seeks answers in the lowest mass stars
What makes a planet?
If you grew up before the mid-2000s, you probably learned that Pluto is a planet, and might have reacted strongly to its subsequent demotion to “dwarf planet” status on August 24th, 2006. Just as the lower limit of what counts as a “planet” proper has changed over time, so too has the upper limit. Planets are defined as a celestial body which (according to the International Astronomical Union’s definitions) has: 1) sufficient mass to collapse into a rounded shape, 2) has sufficiently cleared its orbit of other celestial bodies, and 3) has a mass lower than 13 times the mass of Jupiter, which prevents it from igniting nuclear fusion of any elements. The second qualifier demoted Pluto, a decision which is still hotly debated, considering most “planets” in our solar system, even behemoths like Jupiter, do not have cleared orbits. Qualifier three enjoys its own fair share of scrutiny: what are we to make of objects with masses greater than this “13Mjup limit” that only undergo nuclear fusion for a short portion of their lives? Daniella Baradelz Gagliuffi, a speaker at the recent Physics and Astronomy Department colloquium, seeks answers to these questions by studying brown dwarfs (BDs): celestial objects that bridge the gap between the smallest stars and the most massive planets.
Enter the stellar Twilight Zone
Brown dwarfs are commonly referred to as “failed stars.” Unlike boundaries between dwarf planets and planets, or planets and brown dwarfs, the difference between a brown dwarf and a star is (theoretically) clean cut. Stars are celestial objects that sustain hydrogen fusion at their core. If a celestial body doesn’t form with the sufficient mass (approximately 80 times the mass of Jupiter) to ignite hydrogen burning it can’t be classified as a star. Similarly to how they fail to be stars, BDs also can’t be planets. It’s generally accepted that planets do not undergo nuclear fusion of any kind. At the beginning of their lives, they fuse deuterium into helium, but eventually run out of deuterium and fizzle out.
When prompted to explain the takeaway of her recent colloquia, Bardalez Gagliuffi said “There is this big, open question on how Jupiter formed… there are lots of similarities between low mass brown dwarfs and high mass planets, but… the pivotal difference is how they formed.” To understand how Jupiter formed, it’s necessary to understand how it's similarly appearing, yet compositionally distinct big cousins—brown dwarfs—form.
Image caption: This image from Bardalez Gagliuffi’s presentation, attributed to Michael Cushing (U. Toledo), shows how the temperature of celestial objects of different masses change as they age. Blue lines represent stars that cool briefly after they form but settle into a constant temperature after igniting hydrogen fusion, which serves as a constant heat source. Red lines represent planets, who form and cool continuously over their entire life because they do not undergo any nuclear fusion. Purple lines represent brown dwarfs that cool after formation, level out briefly while fusing deuterium, and then cool again.
In an ideal world, the way to differentiate a planet from a brown dwarf would be to have a measure of its formation process (which takes place over millions of years). Since we can only observe these objects on human timescales, we have to piece together models of how planets and brown dwarfs form by observing lots of them. These observations are difficult to make because of how dim BDs and planets are. They aren’t big and they don’t fuse hydrogen, so they aren’t very bright. Brown dwarfs and planets orbiting nearby stars were only detected in the late 1980’s. Our understanding of these objects is further complicated because in the “absence of an internal energy-generation mechanism results in a degeneracy between mass, age, and luminosity [and their observational proxies]” (Bardalez Gagliuffi, 2019). To put it simply: younger BDs can look like young, massive planets, while old, low mass BDs can resemble older massive planets.
Despite this, the individual and statistical characterization of brown dwarfs is important for understanding the products of stellar and planetary “formation pathways.” In her talk, Bardalez Gagliuffi explains the approaches she’s undertaken to do so.
You may be cool, but you’ll never be Ultracool SpeXtroscopic Survey cool
To constrain the brown dwarf population statistically, Bardalez Gagliuffi has published the results of the Ultracool SpeXtroscopic Survey, a volume limited (every object within a certain distance from the sun is surveyed) spectroscopic survey of very low mass objects on both sides of the hydrogen-burning limit. Two important properties of the BD population for astronomers to constrain are the luminosity function and the binary fraction. “The [luminosity function] LF is a proxy for the mass function (MF). For stars, there is a mass-luminosity relation. For BDs, since they cool over time (the mass-age-luminosity degeneracy), it’s very hard to pin down their ages, and without an age, we can’t figure out a mass. So we use a LF as a proxy,” said Bardalez Gagliuffi in correspondence. Bardalez Gagliuffi further explains, “The [binary fraction] BF is the fraction of multiple systems vs single objects for a given bin of mass or spectral type. In principle, star formation simulations can return this value as a result, so we can compare it against observations. Hence, this statistic is sensitive to the set of assumptions in your formation model.” One of the key takeaways of the survey is that the literature’s sample of objects is incomplete both “in terms of the number of objects identified, and … in the number of binary systems discovered.” This means that our local astronomical neighborhood isn’t as well studied as we might have assumed.
The missing link?
In addition to her survey work, Bardalez Gagliuffi presented a recent discovery and verification of one of the reddest, coldest brown dwarfs ever found. WISE J0830+2837 is a Y-type brown dwarf and has a temperature of ~350K. Unlike other objects Bardalez Gagliuffi discussed this object is special because it falls between the coldest BD ever discovered and the wider brown dwarf population. More nuanced methods of distinguishing between giant planets and brown dwarfs will become necessary as discoveries like this continue to complicate our assumptions about how stars and planets form.
Image caption: This artist's interpretation shows a Y-type brown dwarf hanging out in the vacuum of space. These objects are the coolest class of brown dwarfs, and those most similar to giant planets like Jupiter. Image credit: NASA/JPL-Caltech
Citizen science and community engagement
I first met Daniella last semester before her talk at Amherst was scheduled. We both attended the Inclusive Astronomy 2 (IA2) conference Space Telescope Science Institute (STScI) in Baltimore. There, she presented the COSMOAmautas project, an IAU funded program to train teachers in the Junín region of Peru how to integrate astronomy into the coursework. “I was so excited about the project, I really wanted to present [at IA2],” she told me. COSMOAmautas seeks not only to bring astronomy to public schools in the Peruvian highlands, but to teach astronomy in an engaged, unconventional way. In addition to helping teachers design lectures, the project will scaffold discussion sections, integrate astronomy concepts into math and physics coursework, construct demos, and connect astronomy learning in the classroom to state-of-the-art research. “[Teachers] say, ‘this is science, here. It’s done!’ but it's important to teach students that science is always changing,” she says.
COSMOAmautas mirrors much of Daniella’s other work. In addition to her public interactions at the American Museum of Natural History, including participating in “meet a scientist” days, her discovery of WISE J0830+2837 was aided by the Backyard Worlds: Planet 9 citizen science initiative. This crowdsourced project seeks to identify our own solar systems postulated “Planet 9” while also serving to discover new brown dwarfs. You can participate in the project yourself!
Bardalez Gagliuffi’s work seeks to reckon our rudimentary understanding of star and planet formation with observations of some of the weirdest, label defying celestial bodies in the universe. She provides an incredible working example of a socially engaged, responsible, and community-driven scientist - proving the merit of working collectively to solve some of the most nuanced problems in astronomy.