Brown Dwarf Discovery

The Cool Star Lab team is engaged in the search and characterization of the coldest, oldest, and weirdest cool stars and brown dwarfs in the Milky Way, partnering with citizen scientists through the Backyard Worlds: Planet 9 project

Current Projects

Backyard Worlds: Planet 9

This citizen-science program has uncovered hundreds of new low-mass stars and brown dwarfs from multi-epoch WISE/NEOWISE data. Our team obtained classifying spectra with Lick, Keck, and IRTF observatories.

The Local Sample

Stars and brown dwarfs that are closest to the Sun are the easiest to study through high imaging and spectral resolution methods. Our team targets sources "overlooked" in the literature, obtaining spectra at Lick & Keck observatories that determine classifications, physical properties, and magnetic activity.

High-velocity Neighbors

Using survey data from various surveys, we are studying stars with the highest velocities relative to the Sun, sources that may be low-mass members of the Milky Way's ancient thick disk and halo populations, or have been ejected through dynamical encounters.

Recent Findings

Ancient Stars Discovered in a Near-Infrared Proper Motion Survey

(November 2025) South Pasadena high school researcher Wings Zhang reported the discovery of over 100 new high proper stars detected in the UKIRT Hemisphere Survey (UHS), as part of the Backyard Worlds collaboration. Shane 3m Kast spectroscopy obtained by Cool Star Lab members confirmed two sources as metal-poor M subdwarfs 100-200 pc from the Sun, and color and absolute magnitude measurements indicate these sources encompass a wide range of previously unknown, ancient metal-poor low-mass stars (see the AJ article by Zhang et al.).

Spectra of our Nearest Neighbors

(May 2025) Cool Star Lab undergraduate Marylin Loritsch has led a Research Note characterizing the optical spectra of low-mass stars within 20 pc of the Sun, many of which have not been previously classified. Marylin measured spectral types, metallicity indices, and H-alpha emission, and also identified a previously unrecognized low-mass binary system LP 546-37. Marylin conducted this research as part of the STARTastro program (read the Research Note by Loritsch et al. 2025).

First Ultracool Dwarfs from Euclid

(March 2025) Cool Star Lab members welcomed the first ultracool dwarf datasets from Euclid by contributing to a series of papers from the surveys first quicklook data release (Q1). Zerjal et al. (2025) reports the photometric selection of over 1000 L and T dwarfs in 63 deg2 of deep photometric data, predicting >105 such sources in the final survey. Dominguez-Tagle et al. (2025) present the first analysis of Euclid spectra of ultracoo dwarfs, enabling the template sequence shown above. Both studies were facilitated by SpeX templates provided in the SpeX Prism Library Analysis Toolkit (SPLAT). Euclid will be a major discovery machine for brown dwarfs across the Galaxy over the next few years (read the preprints by Zerjal et al. and Dominguez-Tagle et al.).

Regulus E Discovery Featured on AAS Nova

(February 2025) The discovery of a potential fifth component to the Regulus system recently reported by Eric Mamajek and Adam Burgasser in the Astronomical Journal has been featured on AAS's Nova site! Nova curates the most interesting recent results published in AAS journals, providing astronomy researchers and enthusiasts summaries of recent research across a wide range of astronomical fields. (read the Nova article: https://aasnova.org/2025/02/05/a-new-groupie-in-regulus-entourage/)

A Fifth Member of the Regulus Band

(December 2024) NASA JPL's Eric Mamajek and CSL Director Adam Burgasser have potentially identified a fifth member of the Regulus star system. The discovery, a previously known L/T transition object 2MASS J10071185+1930563 was observed with Keck/NIRES, and the source's radial velocity, distance and proper motion all align with Regulus, suggesting a physical connection or a common origin. Remarkably, the brown dwarf is 7.5 degrees away from Regulus, about 3.9 parsecs (12.6 lightyears) in projected distance, almost 3 times the separation of the Sun and its nearest stellar companion Proxima Centauri. Regulus, or alpha Leonis, is the brightest star in the constellation Leo, and is a binary star composed of a B subgiant and a white dwarf that may have interacted. The other two stars in the system, Regulus BC (aka HD 87884), are a K dwarf/M dwarf pair, making this an extremely wide hierarchical quintuple (read the preprint by Mamajek & Burgasser)

New Ultracool Dwarfs Found with Machine Learning Tools

(November 2024) NAU undergraduate Hunter Brooks and members of the Backyard Worlds: Planet 9 team have uncovered a swarm of ultracool stars and brown dwarfs using machine learning tools. The team used the SMDET neural network code developed by citizen scientist Dan Caselden to find 118 new low-temperature sources in a combination of PanSTARRS, UKIDSS, VISTA, and WISE/CATWISE survey data, and confirmed two objects as new T dwarfs with SpeX infrared spectroscopy. This work paves the way for larger samples constructed and characterized using deep multi-band survey data (read the article in the Astronomical Journal by Brooks et al.)

Cool Stars in the New York Times!

(August 2024) Our speedy little star was featured in the New York Times! NYT Science Reporter Katrina Miller highlighted the contributions of citizen scientists like Tom Bickle who was one of three amateur astronomers to identify the source as part of the Backyard Worlds: Planet 9 project.

A Speedy L Subdwarf

(July 2024) Adam Burgasser led a study on a remarkably fast-moving, metal-poor L dwarf uncovered by citizen scientists associated with the Backyard Worlds: Planet 9 program. The source, CWISE J1249+3621, has speed of 456± 27 km/s in the Milky Way rest frame, placing it near the local Galactic escape velocity. The research team considered several possible origins for the source, including ejection from the center of the Milky Way or globular clusters after interaction with black holes, escape from an exploding Type Ia supernova, and infall from a Milky Way satellite. The result was highlighted in a press conference at AAS 244 (read the article by Burgasser et al. in Astrophysical Journal Letters)

A young and red L dwarf

(May 2024) The Backyard Worlds: Planet 9 collaboration reports the discovery of a new unusually red and young L dwarf. The source, VHS J1831-5513, lies right on the L dwarf/T dwarf transition and is the second reddest brown dwarf identified to date. Its near-infrared spectrum shows evidence of an unusually low surface gravity, and kinematic analysis suggests it is a member of the 22 Myr-old Beta Pictoris association. With an estimated mass of only 6.5±1.5 Jupiter masses, VHS J1831-5513 is a rare "free floating planet" (read the article by Bickle et al. in Astronomical Journal)

Ultracool Dwarfs discovered with Gaia

(May 2024) Adam Burgasser contributed to a study of new nearby ultracool dwarfs identified by Gaia. Near-infrared spectroscopy reveal the majority to be late-M dwarfs and early L dwarfs, with several resolved an unresolved binaries in the sample. This work continues to improve the census of our nearest neighbors (read the A&A paper by Ravinet et al.)

New Brown Dwarf Benchmarks

(April 2024) Cool Star Lab members contributed to the discovery of 13 new M dwarf + T dwarf wide binaries identified in CATWISE2020 and Backyard Worlds data. These important benchmark systems enable better understanding of the influence of age and metallicity on the evolution and atmospheres of low-temperature brown dwarfs. The sample includes at least one potentially young T dwarf that could have a mass as low as 2 Jupiter masses (read the article by Marocco et al. in Astrophysical Journal)

Building the census of the Solar Neighborhood

(April 2024) Cool Star Lab members contributed to a detailed census of the local Solar Neighborhood. The volume-limited census of about 3600 individual objects, including components of multiple systems, extends from giant "planets" (≈ 5 Jupiter masses) to massive stars (≈8 solar masses). This comprehensive study reports the most accurate measurement of the stellar initial mass function to date, and finds that roughly 20% of all "stars" are substellar brown dwarfs. (read the ApJS article by Kirkpatrick et al.)

A Bevy of Brown Dwarf Companions

(March 2024) Cool Star Lab members contributed to a massive haul of benchmark ultracool dwarf companions to nearby stars identified by the Backyard Worlds Team. CUNY graduate student Austin Rothermich identified 89 new systems with G2-M9 primaries and M7-T9 companions with diverse spectral properties, and many of these systems appear to be triples and quadruples. These systems span exceptionally wide separations and low mass ratios, in the latter case with similar values as exoplanet systems. (read the preprint by Rothermich et al. and watch his AAS 243 press conference)

An unusual and cold brown dwarf

(Oct 2023) Cool Star Lab members contributed to the discovery of a new Y dwarf identified by the Backyard Worlds Team. The source, CWISE J105512.11+544328.3, was confirmed and classified with Keck/NIRES near-infrared spectroscopy, and has an estimated temperature of 500 K. It's extremely blue mid-infrared color suggests it may have an unusual, possibly metal-poor atmosphere. The publication was led by U. Florida undergraduate Grady Robbins (see the preprint by Robbins et al.)

A Wide Pair of L dwarfs Found by Citizen Scientists

(Aug 2023): The Cool Star Lab obtained Keck/NIRES and Keck/NIRSPEC data for a newly-resolved L dwarf binary pair identified by citizen scientists in the Backyard Worlds: Planet 9 program. CWISE J0617+1945AB is an L2 + L4 pair at 28 pc separated by 1.3 arcseconds. Its wide separation makes it an important benchmark for comparative L dwarf studies (read the Research Note by Humphries et al.).

Finding benchmark ultracool dwarfs

(Mar 2023): Cool Star Lab undergraduates Tianxing Zhou, Delilah Jacobsen, and Brigette Vazquez-Segovia have reported a new sample of benchmark ultracool dwarfs, based on co-moving systems identified by Gaia. These include 100 systems within 100 pc of the Sun that currently lack spectroscopic characterization (read the paper by Zhou et al. at AAS Research Notes)

Discovery of a young planetary mass object

(Jan 2023): The Cool Star Lab contributed to the discovery of an extremely red L/T dwarf by the Backyard Worlds program, which is likely a planetary mass object in the 22 Myr-old Beta Pic moving group. The CSL team obtained the spectrum of this unique young source with the NIRES spectrograph at Keck Observatory (read the paper by Schneider et al. in ApJ Letters).

The M dwarf companion to µ Virgenes

(Dec 2022) Summer research student and Lamat Institute scholar Julissa Villalobos Valencia led a spectroscopic study of the M5 dwarf companion to the bright star µ Virgenes, which is visible to the naked eye (read the paper by Villalobos Valencia et al. at AAS Research Notes)

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