Science of SPARCS

Low-mass main sequence stars of spectral type M and K are the primary stellar constituents of the Galaxy, with stellar masses ranging from about 10% to 80% of the mass of the Sun. Statistical studies based on the results of exoplanet-hunting survey missions estimate that fifty billion of these stars host at least one small planet within their habitable zone (HZ) — the narrow region around a star where temperatures allow for liquid surface water. Given their large population and the high frequency of planets orbiting them, these red dwarfs represent some of the most promising targets in the search for habitable Earth-like exoplanets.

However, the habitability of red dwarf systems is complicated by the powerful and highly variable ultraviolet (UV) radiation from the host stars. These stars are known to remain active for much of their lifetimes, maintaining elevated UV emission levels and frequent flaring for up to a billion years, particularly for the lowest-mass M stars. This radiation plays a critical role in shaping planetary atmospheres by driving atmospheric loss, altering chemical composition, and influencing overall habitability. UV photons are capable of dissociating atmospheric molecules, fundamentally changing atmospheric chemistry and potentially masking or mimicking the spectral biosignatures used to evaluate planetary habitability.

Stellar UV radiation strongly impacts key planetary atmosphere constituents such as water, ozone, sulfur dioxide (a tracer of volcanic activity), and ammonia (an essential precursor for amino acid synthesis). This impact is even stronger for planets orbiting K and M dwarfs due to their compact habitable zones, which lie just 0.1 to 0.4 AU from the host star. Furthermore, these low-mass stars exhibit far-UV (FUV) to near-UV (NUV) flux ratios that exceed the Sun's by two to three orders of magnitude. Such a spectral energy distribution leads to enhanced production of oxygen and ozone in planetary atmospheres. While this can influence habitability, it may also generate false-positive biosignatures, complicating the interpretation of exoplanet observations.

Despite the importance of UV radiation, the characteristics of UV flares on low-mass stars —specifically their occurrence rate, energy distribution, and long-term evolution — are not well constrained. UV emission does not reach the Earth's surface, such that low-mass star UV flare studies have to rely on space-based observatories. While legacy space-based missions such as the International Ultraviolet Explorer (IUE), the Galaxy Evolution Explorer (GALEX), and the Hubble Space Telescope (HST) have provided valuable insights, none were designed to systematically study the long-term UV behavior of low-mass stars. Existing observations typically consist of short exposures lasting minutes to hours. This lack of long-term continuous monitoring makes it difficult to statistically quantify rare, high-energy flares or understand long-term variability.

To address this observational gap, the Star-Planet Activity Research CubeSat (SPARCS) has been developed as the first mission dedicated to monitoring the time-dependent UV radiation of low-mass stars. SPARCS is a 6U CubeSat (30 cm × 20 cm × 10 cm) that will monitor low-mass stars in two ultraviolet bands: the near-UV (NUV; 260-300 nm, centered on the Mg II emission line in low-mass stars) and the far-UV (FUV; 153-171 nm, encompassing the C IV and He II emission lines). These wavelength ranges are highly sensitive to stellar flares and are vital for modeling the evolution of exoplanetary atmospheres and quantifying atmospheric escape rates on orbiting planets.

SPARCS is designed to probe both short- and long-term stellar UV variability, capturing everything from rapid flaring events to slow modulations of the quiescent (non-flaring) emission. The SPARCS NUV band probes chromospheric variability, while the FUV band looks into events occurring in the transition region, thus offering critical empirical data for refining stellar activity models. These models will, in turn, be used to predict exoplanet atmospheric spectra and help better interpret spectroscopic observations conducted by facilities like the James Webb Space Telescope (JWST).

During its one-year primary mission, SPARCS is expected to observe 20 low-mass stars spanning spectral types K0 to M5, including both active and inactive stars, as well as young and old systems. Target brightness ranges from approximately 1 mJy to 16 Jy at 280 nm.

SPARCS is expected to be inserted into a sun-synchronous orbit, which will enable near-continuous monitoring of each target for durations of 5 to 40 days, covering one to three full stellar rotation periods. This observing strategy allows SPARCS to measure slow rotational modulation of the quiescent stellar flux due to surface spot activity, as well as to detect and characterize stellar flares across a wide range of energies. By increasing cumulative UV exposure time by roughly three orders of magnitude compared to existing archival data, the mission will establish robust UV flare frequency distributions (FFDs) and record the rare, high-energy events that have escaped detection by previous short-duration observations.

Beyond its scientific goals, SPARCS also serves as a key technology demonstrator for next-generation ultraviolet astronomy. The SPARCS science instrument uses a UV camera (SPARCam) incorporating delta-doped CCD detectors developed at the Jet Propulsion Laboratory. These detectors provide nearly 100% internal quantum efficiency (QE) throughout the UV range and feature integrated bandpass filters optimized for FUV observations. By validating these high-performance detectors in a space environment, SPARCS will establish the technical readiness required for future flagship initiatives, most notably the Habitable Worlds Observatory.

Through its science, technology development, and educational impact, SPARCS plays a pivotal role in advancing our understanding of stellar activity, exoplanet environments, and the long-term habitability of planets orbiting low-mass stars.