Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as ‘ultracool dwarfs’1. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun2. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disks3, 4, there should be a large but hitherto undetected population of terrestrial planets orbiting them5—ranging from metal-rich Mercury-sized planets6 to more hospitable volatile-rich Earth-sized planets7. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star8. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system.
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: Raw TRAPPIST-1 transit light curves. (507 KB)
The light curves are shown in chronological order from top to bottom and left to right, with unbinned data shown as cyan dots, and binned 0.005-day (7.2-minute) intervals shown as black dots with error bars. The error bars are the standard errors of the mean of the measurements in the bins. The best-fit transit-plus-baseline models are overplotted (red line). The light curves are phased for the mid-transit time and shifted along the y axis for clarity. For the dual transit of 11 December 2015, the light curve is phased for the mid-transit time of planet TRAPPIST-1c. T1b, TRAPPIST-1b; T1c, TRAPPIST-c; T1d, TRAPPIST-1d.
- Extended Data Figure 2: De-trended TRAPPIST-1 transit light curves. (487 KB)
The details are as in Extended Data Fig. 1, except that the light curves shown here are divided by the best-fit baseline model to highlight the transit signatures.
- Extended Data Figure 3: Near-infrared spectra of TRAPPIST-1. (256 KB)
a, Comparison of TRAPPIST-1’s near-infrared spectrum (black)—obtained with the spectrograph IRTF/SpeX35—with that of the M8-type standard LHS132 (red). b, Cross-dispersed IRTF/SpeX spectrum of TRAPPIST-1 in the 2.17–2.35-μm region. Na i, Ca i and CO features are labelled. Additional structure primarily originates from overlapping H2O bands. The spectrum is normalized at 2.2 μm. Fλ, spectral flux density; fλ, normalized spectra flux density.
- Extended Data Figure 4: Flare events in the TRAPPIST 2015 photometry. (294 KB)
The photometric measurements are shown unbinned (cyan dots) and binned per 7.2-minute interval (black dots). For each interval, the error bars are the standard error of the mean.
- Extended Data Figure 5: Photometric variability of TRAPPIST-1. (374 KB)
a, Global light curve of the star as measured by TRAPPIST. The photometric measurements are shown unbinned (cyan dots) and binned per night (black dots with error bars (±s.e.m.)). This light curve is compared with that of the comparison star 2MASS J23063445 − 0507511, shifted along the y axis for clarity. b, The same light curve for TRAPPIST-1, folded on the period P = 1.40 days and binned by 10-minute intervals (error bars indicate ±s.e.m.). For clarity, two consecutive periods are shown.