Since it takes less illumination to reach the same temperatures, the habitable zone shifts further away from the star. The offset between the two zones is due to the cooler TRAPPIST-1 star emitting more of its light in the form of infrared radiation that is more efficiently absorbed by an Earth-like atmosphere. All of the known TRAPPIST-1 planets are larger than Mars, with 5 of them within 15% of the diameter of the Earth.The corresponding "habitable zones" of the two planetary systems, regions where an Earth-like planet could potentially support liquid water on its surface, are indicated near the top of the plot. The relative sizes of the planets are indicated by the circles. This graph presents measured properties of the seven TRAPPIST-1 exoplanets (labeled b through h), showing how they stack up to each other as well as to Earth and the other inner rocky worlds in our own solar system. NASA's Hubble and now-retired Kepler space telescopes have also studied the system.Īll seven TRAPPIST-1 planets, which are so close to their star that they would fit within the orbit of Mercury, were found via the transit method: Scientists can't see the planets directly (they're too small and faint relative to the star), so they look for dips in the star's brightness created when the planets cross in front of it. Managed by NASA's Jet Propulsion Laboratory in Southern California, Spitzer observed the system for over 1,000 hours before being decommissioned in January 2020. Subsequent observations by NASA's now-retired Spitzer Space Telescope, in collaboration with ground-based telescopes, confirmed two of the original planets and discovered five more. Some of these planets have been known since 2016, when scientists announced that they'd found three planets around the TRAPPIST-1 star using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. Based on that conclusion, the paper authors hypothesized a few different mixtures of ingredients could give the TRAPPIST-1 planets the measured density. But if this is the case, that ratio must be notably different than Earth's: The TRAPPIST-1 planets are about 8% less dense than they would be if they had the same makeup as our home planet. That could mean they all contain about the same ratio of materials thought to compose most rocky planets, like iron, oxygen, magnesium, and silicon. Located about 40 light-years away, these seven rocky siblings provide an example of the tremendous variety of planetary systems that likely fill the universe.Ī new study published today in the Planetary Science Journal shows that the TRAPPIST-1 planets have remarkably similar densities. The red dwarf star TRAPPIST-1 is home to the largest group of roughly Earth-size planets ever found in a single stellar system. Density measurements are a critical first step in determining the composition and structure of exoplanets, but they must be interpreted through the lens of scientific models of planetary structure. The high precision mass and diameter measurements of the exoplanets in the TRAPPIST-1 system have allowed astronomers to calculate the overall densities of these worlds to an unprecedented degree of accuracy in exoplanet research. Since all seven planets have remarkably similar densities, it is more likely that all the planets share a similar bulk composition, making this fourth scenario unlikely, but not ruled out. On the inner three planets, any oceans would vaporize due to the higher temperatures near their star, and a different composition model is required. However, this scenario can only be applied to the outer four planets in the TRAPPIST-1 system. The second model shows an overall composition similar to Earth, in which the densest materials have settled to the center of the planet, forming an iron-rich core proportionally smaller than Earth's core.A variation is shown in the third panel where a larger, denser core could be balanced by an extensive low density ocean on the planet's surface. There is no solid iron core, as is the case with Earth and the other rocky planets in our own solar system. The planet densities are slightly lower than that of Earth or Venus, which could mean they contain fractionally less iron (a highly dense material), or more low-density materials, such as water or oxygen.In the first model (left), the interior of the planet is composed of iron mixed with lighter elements, such as oxygen. Overall the TRAPPIST-1 worlds have remarkably similar densities, which suggests they may share the same ratio of common planet-forming elements. This illustration shows three possible interiors of the seven rocky exoplanets in the TRAPPIST-1 system, based on precision measurements of the planet densities.
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