Temperate Earth-sized planets transiting a nearby ultracool dwarf star

Journal name:
Nature
Volume:
533,
Pages:
221–224
Date published:
DOI:
doi:10.1038/nature17448
Received
Accepted
Published online

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.

At a glance

Figures

  1. Transit photometry of the TRAPPIST-1 planets.
    Figure 1: Transit photometry of the TRAPPIST-1 planets.

    Each light curve is phased to the time of inferior conjunction (mid-transit time) of the object. The light curves are binned in two-minute intervals for planet TRAPPIST-1b (a), and in five-minute intervals for planets TRAPPIST-1c (b) and TRAPPIST-1d (c). The best-fit transit models, as derived from a global analysis of the data, are overplotted (red lines). The light curves are shifted along the y axis for the sake of clarity. For the HCT/Hanle faint object spectrograph camera (HFOSC) light curve, the data are unbinned and the error bars are the formal measurement errors. For the other light curves, the error bars are the standard errors of the mean of the measurements in the bin. WFCAM, wide-field infrared camera on the UKIRT; HAWK-I, high acuity wide field K-band imager on the VLT.

  2. Masses of host stars and equilibrium temperatures of known sub-Neptune-sized exoplanets.
    Figure 2: Masses of host stars and equilibrium temperatures of known sub-Neptune-sized exoplanets.

    The size of the symbols scales linearly with the radius of the planet. The background is colour-coded according to stellar mass (in units of the Sun’s mass). The TRAPPIST-1 planets are at the boundary between planets associated with hydrogen-burning stars and planets associated with brown dwarfs. Equilibrium temperatures are estimated neglecting atmospheric effects and assuming an Earth-like albedo of 0.3. The positions of the Solar System terrestrial planets are shown for reference. The range of possible equilibrium temperatures of TRAPPIST-1d is represented by a solid bar; the dot indicates the most likely temperature. Only the exoplanets with a measured radius equal to or smaller than that of GJ 1214b are included.

  3. Potential for characterizing the atmospheres of known transiting sub-Neptune-sized exoplanets.
    Figure 3: Potential for characterizing the atmospheres of known transiting sub-Neptune-sized exoplanets.

    The signal being transmitted from each planet is estimated in parts per million (p.p.m.) and for transparent water-dominated atmospheres with a mean molecular weight, μ, of 19. The signal-to-noise ratio (SNR) in transmission (normalized to that of GJ 1214b under the same atmospheric assumptions) is also calculated. The estimated signal and SNR are plotted against equilibrium temperatures, assuming a Bond albedo of 0.3. The black horizontal bar indicates the SNR that will require 200 (or 500) [or 1,000] hours of in-transit observations with the James Webb Space Telescope to yield a planet’s atmospheric temperature with a relative uncertainty below 15% and with abundances within a factor of four in the case of a H2O (or N2) [or CO2]-dominated atmosphere (μ = 19 (or 28) [or 39]). Only the exoplanets with a measured radius equal to or smaller than that of GJ 1214b are included in the figure. The size of the circular symbol for each planet is proportional to the planet’s physical size. For illustration, symbols for planets of one (R) and two (2R) Earth radii are shown at the top right.

  4. Raw TRAPPIST-1 transit light curves.
    Extended Data Fig. 1: Raw TRAPPIST-1 transit light curves.

    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.

  5. De-trended TRAPPIST-1 transit light curves.
    Extended Data Fig. 2: De-trended TRAPPIST-1 transit light curves.

    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.

  6. Near-infrared spectra of TRAPPIST-1.
    Extended Data Fig. 3: Near-infrared spectra of TRAPPIST-1.

    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.

  7. Flare events in the TRAPPIST 2015 photometry.
    Extended Data Fig. 4: Flare events in the TRAPPIST 2015 photometry.

    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.

  8. Photometric variability of TRAPPIST-1.
    Extended Data Fig. 5: Photometric variability of TRAPPIST-1.

    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.

Tables

  1. TRAPPIST-1 transit light curves
    Extended Data Table 1: TRAPPIST-1 transit light curves
  2. Quadratic limb-darkening coefficients
    Extended Data Table 2: Quadratic limb-darkening coefficients
  3. Posterior likelihoods of the orbital solutions for TRAPPIST-1d
    Extended Data Table 3: Posterior likelihoods of the orbital solutions for TRAPPIST-1d
  4. Individual mid-transit timings measured for the TRAPPIST-1 planets
    Extended Data Table 4: Individual mid-transit timings measured for the TRAPPIST-1 planets

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Author information

Affiliations

  1. Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium

    • Michaël Gillon,
    • Emmanuël Jehin,
    • Laetitia Delrez,
    • Artem Burdanov,
    • Valérie Van Grootel,
    • Cyrielle Opitom &
    • Pierre Magain
  2. NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas, 77058, USA

    • Susan M. Lederer
  3. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Julien de Wit
  4. Center for Astrophysics and Space Science, University of California San Diego, La Jolla, California 92093, USA

    • Adam J. Burgasser &
    • Daniella Bardalez Gagliuffi
  5. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

    • Amaury H. M. J. Triaud
  6. Astrophysics Group, Cavendish Laboratory, 19 J J Thomson Avenue, Cambridge, CB3 0HE, UK

    • Brice-Olivier Demory &
    • Didier Queloz
  7. Indian Institute of Astrophysics, Koramangala, Bangalore 560 034, India

    • Devendra K. Sahu

Contributions

The TRAPPIST team (M.G., E.J., L.D., A.B., C.O. and P.M.) discovered the planets. M.G. leads the exoplanet program of TRAPPIST, set up and organized the ultracool-dwarf transit survey, planned and analysed part of the observations, led their scientific exploitation, and wrote most of the manuscript. E.J. manages the maintenance and operations of the TRAPPIST telescope. S.M.L. obtained the director’s discretionary time on UKIRT, and managed, with E.J., the preparation of the UKIRT observations. L.D. and C.O. scheduled and carried out some of the TRAPPIST observations. L.D. and A.B. analysed some photometric observations. J.d.W. led the study of the amenability of the planets for detailed atmospheric characterization. V.V.G. checked the physical parameters of the star. A.J.B. checked the spectral type of the star and determined its metallicity. B.-O.D. took charge of the dynamical simulations. D.B.G. acquired the SpeX spectra. D.K.S. gathered the HCT observations. S.M.L., A.H.M.J.T., P.M. and D.Q. helped to write the manuscript. A.H.M.J.T. prepared most of the figures.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

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Author details

Extended data figures and tables

Extended Data Figures

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

Extended Data Tables

  1. Extended Data Table 1: TRAPPIST-1 transit light curves (463 KB)
  2. Extended Data Table 2: Quadratic limb-darkening coefficients (157 KB)
  3. Extended Data Table 3: Posterior likelihoods of the orbital solutions for TRAPPIST-1d (253 KB)
  4. Extended Data Table 4: Individual mid-transit timings measured for the TRAPPIST-1 planets (544 KB)

Additional data