Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1

Abstract

One aim of modern astronomy is to detect temperate, Earth-like exoplanets that are well suited for atmospheric characterization. Recently, three Earth-sized planets were detected that transit (that is, pass in front of) a star with a mass just eight per cent that of the Sun, located 12 parsecs away. The transiting configuration of these planets, combined with the Jupiter-like size of their host star-named TRAPPIST-1-makes possible in-depth studies of their atmospheric properties with present-day and future astronomical facilities. Here we report the results of a photometric monitoring campaign of that star from the ground and space. Our observations reveal that at least seven planets with sizes and masses similar to those of Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain, such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.1 and 12.35 days) are near-ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inwards. Moreover, the seven planets have equilibrium temperatures low enough to make possible the presence of liquid water on their surfaces.

Extended Data Figure 1. Light curve of a triple transit of planets c-e-f

The black points show the differential photometric measurements extracted from VLT/HAWK-I images, with the formal 1-sigma errors shown as vertical lines. The best-fit triple transit model is shown as a red line. Possible configurations of the planets relative to the stellar disc are shown below the light curve for three different times (red = planet c, yellow = planet e, green = planet f). The relative positions and sizes of the planets, as well as the impact parameters correspond to the values given in Table 1.

Extended Data Figure 2. Transit light curve of TRAPPIST-1d and e

The black points show the photometric measurements - binned per 0.005d = 7.2min. The error for each bin (shown as vertical line) was computed as the 1-sigma error on the average. These light curves are divided by their best-fit instrumental models and by the best-fit transit models of other planets (for multiple transits). The best-fit transit models are shown as solid lines. The light curves are period-folded on the best-fit transit ephemeris given in Table 1, their relative shifts on the x-axis reflecting TTVs due to planet-planet interactions (see text). The epoch of the transit and the facility used to observe it are mentionned above each light curve.

Extended Data Figure 3

Transit light curves of TRAPPIST-1f and g. Same as Extended Data Fig. 2 for the planets f and g.

Extended Data Figure 4. Transit Timing Variations (TTVs) measured for TRAPPIST-1b-c-d-e-f-g

For each planet, the best-fit TTV model computed with the N-body numerical integration code Mercury is shown as a red line. The 1-sigma errors of the transit timing measurements are show as vertical lines.

Figure 1. The TRAPPIST-1 system as seen by Spitzer.

a and b. Spitzer photometric measurements (dark points) resulting from the nearly-continuous observation of the star from 19 September to 10 October 2016. The ground-based measurements (binned per 5 min for clarity) gathered during the Spitzer gaps are shown as light grey points. The position of the transits of the planets are shown as coloured diamonds. c. Period-folded photometric measurements obtained by Spitzer near transits of planets TRAPPIST-1b-h corrected for the measured TTVs. Coloured dots show the unbinned measurements, whereas the open circle depict binned measurements for visual clarity. The 1-sigma error bars of the binned measurements are shown as vertical lines. The best-fit transit models are shown as coloured lines. 16-11-5-2-3-2-1 transits were observed by Spitzer and combined to produce the shown light curves for planets b-c-d-e-f-g-h, respectively. d. Representation of the orbits of the 7 planets. The same colour code as in the two other panels is used to identify the planets. The grey annulus and the two dashed lines represent the zone around the star where abundant long-lived liquid water (i.e. oceans) could exist on the surfaces of Earth-like planets as estimated under two different assumptions in ref. 6. The relative positions of the planets corresponds to their orbital phase during the first transit we detected on this star, by TRAPPIST-1c (the observer is located on the right hand-side of the plot).

Figure 2. Mass-radius and incident flux-radius diagrams for terrestrial planets.

In both panels, the coloured circular symbols are the TRAPPIST-1 planets, and the horizontal and vertical lines are 1-sigma error bars. a. Mass-radius relation for planets between 0.5 and 1.5 Earth radii, and between 0.1 and 2 Earth masses. The solid lines are theoretical mass-radius curves for planets with different compositions. The fiducial model is 100% MgSiO3 (rock), whose fractional part is decreasing either with increasing fraction of water (the radius increases), or with increasing fractions of Iron (the radius decreases). b. Radius vs incident flux. Venus and Earth are shown as grey circular symbols, and Mercury, Mars, and Ceres as dotted vertical lines. The planet h has large errors on its irradiation because of its unknown orbital period.