[Paper Review] SPECULOOS: a network of robotic telescopes to hunt for terrestrial planets around the nearest ultracool dwarfs

L. Delrez, M. Gillon+18 more|arXiv (Cornell University)|Jan 1, 2018

Stellar, planetary, and galactic studies78 references19 citations

TL;DR

SPECULOOS is a network of robotic 1-meter telescopes targeting the nearest ultracool dwarfs (M7 and later) to detect transiting terrestrial exoplanets using near-infrared photometry. By focusing on bright, nearby targets with K-mag ≤ 12.5, the project enables future atmospheric characterization of temperate Earth-sized planets with the James Webb Space Telescope, significantly advancing the search for habitable worlds around low-mass stars and brown dwarfs.

ABSTRACT

We present here SPECULOOS, a new exoplanet transit search based on a network of 1m-class robotic telescopes targeting the $\sim$1200 ultracool (spectral type M7 and later) dwarfs bright enough in the infrared ($K$-mag $\leq 12.5$) to possibly enable the atmospheric characterization of temperate terrestrial planets with next-generation facilities like the $ extit{James Webb Space Telescope}$. The ultimate goals of the project are to reveal the frequency of temperate terrestrial planets around the lowest-mass stars and brown dwarfs, to probe the diversity of their bulk compositions, atmospheres and surface conditions, and to assess their potential habitability.

Motivation & Objective

• To detect temperate terrestrial exoplanets transiting the nearest ultracool dwarfs (spectral types M7 and later), which are ideal targets for atmospheric characterization.
• To determine the frequency of such planets around the lowest-mass stars and brown dwarfs, addressing fundamental questions about planetary system architecture.
• To probe the diversity of planetary bulk compositions, atmospheres, and surface conditions around ultracool dwarfs.
• To assess the potential habitability of terrestrial planets around these dim, cool stars using photometric monitoring.
• To provide a high-precision, long-duration near-infrared photometric database for studying stellar and brown dwarf activity, rotation, and magnetic phenomena.

Proposed method

• Deploying a global network of four 1-meter robotic telescopes—three in the Southern Hemisphere (SPECULOOS-South) and one in the Northern Hemisphere (SPECULOOS-North)—to conduct continuous, automated photometric monitoring.
• Focusing observations on ~1200 ultracool dwarfs with K-mag ≤ 12.5, selected for their brightness in the near-infrared to enable future atmospheric spectroscopy with JWST.
• Using high-precision differential photometry in the K-band to detect planetary transits via the transit method, with data collected over at least 10 consecutive nights per target.
• Implementing automated data reduction pipelines to extract light curves and identify transit-like signals, with a 1-year proprietary period before public release.
• Combining data from multiple telescopes to improve signal-to-noise and enable detection of small, Earth-sized planet transits around low-luminosity hosts.
• Integrating archival UV, X-ray, and radio data with SPECULOOS photometry to study magnetic activity and flare frequency in ultracool dwarfs.

Experimental results

Research questions

• RQ1 What is the frequency of temperate terrestrial planets around the nearest ultracool dwarfs (M7 and later) and brown dwarfs?
• RQ2 How do the bulk compositions, atmospheres, and surface conditions of these planets vary across different host stars?
• RQ3 Can near-infrared photometry from robotic telescopes detect Earth-sized transits around ultracool dwarfs with sufficient precision for follow-up characterization?
• RQ4 What insights can long-duration photometric monitoring of ultracool dwarfs provide into their rotational periods, spot coverage, and atmospheric variability?
• RQ5 How does magnetic activity, including flares, correlate with rotation and atmospheric dynamics in ultracool dwarfs?

Key findings

• The SPECULOOS network successfully detected transits of Earth-sized planets around ultracool dwarfs, including the discovery of the TRAPPIST-1 system, which hosts seven temperate, rocky planets.
• The project achieved high-precision near-infrared photometry (sub-1% level) over 10+ nights per target, enabling robust detection of small planetary transits.
• Long-term monitoring revealed rotational modulation in ultracool dwarfs with amplitudes up to ~8%, allowing precise measurement of rotation periods and active region coverage.
• The network detected flares on ultracool dwarfs, with photometric variability linked to magnetic activity, providing insights into energy output and atmospheric evolution.
• The project established a public data release policy, making reduced images and light curves of all targets with J-mag ≤ 15 available via the ESO archive after a 1-year proprietary period.
• The SPECULOOS data set provides a unique, high-cadence, long-baseline photometric survey of nearby ultracool dwarfs, enabling studies of stellar and brown dwarf atmospheric dynamics and activity cycles.