Pinning Down Properties of TRAPPIST-1


TRAPPIST-1, a nearby ultracool dwarf star, was catapulted into the public eye roughly a year ago when it was determined to host seven transiting, Earth-sized planets — three of which are located in its habitable zone. But how correct are the properties we’ve measured for this system?

Sun vs. TRAPPIST-1

TRAPPIST-1 is a very small, dim star — it’s only 11% the diameter of the Sun — which makes it easier for us to learn about its planets from transit data. [ESO]

Intrigue of TRAPPIST-1

One reason the TRAPPIST-1 system is of particular interest to scientists is that its small star (roughly the size of Jupiter) means that the system has a very favorable planet-to-star ratio. This makes it possible to learn a great deal about the properties of the planets using current and next-generation telescopes.

The observations we expect to be able to make of TRAPPIST-1 exoplanets — of the planet atmospheres, surface conditions, and internal compositions, for example — will allow us to test planet formation and evolution theories and assess the prospects of habitability for Earth-sized planets orbiting cool M dwarfs.

Why Stellar Measurements Matter

parallax of TRAPPIST-1

The parallax motion of TRAPPIST-1 in dec (top) and R.A. (bottom) as a function of day. Observations were made between 2013 and 2016 and then folded over a year. [Van Grootel et al. 2018]

In order to make these measurements, however, we first need very precise measurements of the host star’s parameters. This is because transiting exoplanet parameters are generally determined relative to those of the host. A few examples:

  • Determining how much irradiation a planet receives requires knowing the luminosity of the host star and planet’s orbit size. The latter is calculated based on the host star’s mass.
  • Determining the planet’s radius requires knowing the host star’s radius, as the planet’s transit depth tells us only the star-to-planet radius ratio.
  • Determining whether or not the planet is able to retain an atmosphere — and therefore whether it has exhibited long-term habitability — requires knowing the time the host star takes to contract onto the main sequence, which depends on the star’s mass.

When the TRAPPIST-1 planetary system was discovered, measurements of TRAPPIST-1’s properties were made to the best of our abilities at the time. Now, in a new study led by Valérie Van Grootel (University of Liège, Belgium), a team of scientists has used new observations and analysis techniques to refine our measurements of the star.

luminosity vs. age

Stellar luminosity for evolution models for various masses and metallicities. The green dashed horizontal lines bracket the authors’ observed value for TRAPPIST-1’s luminosity. A stellar mass of ~0.09 M is needed to account for the old age and luminosity of the star. [Van Grootel et al. 2018]

New Estimates

Using 188 epochs of observations of TRAPPIST-1 from multiple telescopes between 2013 and 2016, Van Grootel and collaborators obtained a very precise measurement for TRAPPIST-1’s parallax. This allowed them to refine the estimate of its luminosity — now measured at (5.22 ± 0.19) x 10-4 that of the Sun — to twice the precision of the previous estimate.

The team then produced a new estimate for TRAPPIST-1’s mass using new stellar evolution modeling and analysis, combined with empirical mass derived for similar ultracool dwarfs in astrometric binaries. This approach produces a final mass for TRAPPIST-1 of 0.089 ± 0.006 M — which is nearly 10% higher than the previous estimate and significantly more precise. Finally, the authors use these values to obtain new estimates of TRAPPIST-1’s radius (0.121 ± 0.003 R) and effective temperature (2516 ± 41 K).

These new, refined measurements will ensure that our future observations of the TRAPPIST-1 planets are being interpreted correctly — which is critical for a system that will be so thoroughly scrutinized in coming years. Keep an eye out for new results about TRAPPIST-1 in the future!


Valérie Van Grootel et al 2018 ApJ 853 30. doi:10.3847/1538-4357/aaa023