News release | Research | Science
January 22, 2021
Depiction of TRAPPIST-1 star and its seven world actors.NASA / JPL-Caltech / R. Injury (IPAC)
The TRAPPIST-1 star system is home to the largest batch of nearly Earth-sized planets found outside our solar system. Discovered in 2016, some 40 light-years away, these seven rocky siblings offer a glimpse at the tremendous diversity of planets that fill the universe.
A study accepted by the Journal of Planetary Science suggests that planets share the same density. This may mean that they all have the same proportion of material for rocky planets, such as iron, oxygen, magnesium, and silicon. If so, while TRAPPIST-1 planets may resemble each other, they appear differently from Earth: they are about 8% less dense if they have the same chemical composition as our planet.
These findings give astronomers new data, which they try to pin down the exact structure of these planets, and compare them not only to the Earth, but to all the rocky planets in our solar system, by lead author Eric Agol According, Professor of Astronomy of the University of Washington.
“This is the most accurate characterization of a set of rocky exoplanets, which gave us high-confidence measurements of their diameter, density, and mass,” Agol said. “This is the information we need to hypothesize about their creation and understand how these planets differ from the rocky planets in our solar system.”
Since the early detection of the TRAPPIST-1 world in 2016, scientists have studied this planetary family with several space and ground-based telescopes, including NASA’s now-retired Kepler Space Telescope and Spitzer Space Telescope. Spitzer alone provided more than 1,000 hours of targeted observations of the system before it was demoted in January 2020. Since they are too small and too faint to see directly, all seven exoplanets were found via the so-called transit method: glow when the planets are exposed to looking for star dips.
Previous calculations had shown that the planets are roughly the size and mass of the Earth and thus should also be rocky, or terrestrial – as opposed to gas-dominated worlds such as Jupiter and Saturn. This new study provides the most accurate density measurement for any group of exoplanets.
The density of a planet is determined not only by its composition, but also by its size: gravity constricts the planet that is made up of a planet, increasing the density of the planet. Uncompressed density adjusts for the effect of gravity, and can tell how the composition of different planets compares.NASA / JPL-Caltech
“The night sky is full of planets, and it is only within the last 30 years that we have been able to uncover their secrets,” said Caroline Dorn, co-author of the University of Zurich. “The TRAPPIST-1 system is attractive because around this one star we can learn about the diversity of rocky planets within a single system. And we can actually learn more about a planet by studying it with our neighbors, so this system is perfect for that. “
The team – which includes scientists based in the United States, Switzerland, France, the United Kingdom and Morocco – used precise measurements of the time of star dips and planetary orbits to make detailed measurements of each planet’s mass and diameter. And from there to determine its density. Agol and UW co-authors Zachary Langford and Victoria Meadows, a professor of astronomy, analyzed the data and performed computer simulations that constrained the orbits of the TRAPPIST-1 planets and calculated their densities.
With more accurate measurement of the density of an object, we can learn more about its structure. A baseball and paperweight can be the same size, but the baseball is much lighter. Width and weight together reveal the density of each object, and from there it is possible to infer that a baseball is made of lightweight materials like string and leather, while paperweight has a heavier structure, such as glass or metal.
In our own solar system, the density of the eight planets varies widely. Gas giants – Jupiter, Saturn, Uranus and Neptune – are larger, but much less dense than the four rocky planets. Earth, Venus and Mars have similar densities, but Mercury has a higher percentage of iron, so although it is the smallest planet in the Solar System in diameter, Mercury has the second highest density among all eight planets.
Seven TRAPPIST-1 planets, on the other hand, all share the same density, which makes the system significantly different from our own. The difference in density between TRAPPIST-1 planets and Earth, Venus and Mars, may seem small – about 8% – but it is significant on a planetary scale. For example, one way to explain low density is that the TRAPPIST-1 planets have the same structure as the Earth, but with a lower percentage of iron – about 21% compared to the Earth’s 32%, according to the study.
Alternatively, iron in TRAPPIST-1 planets can be infected with a high level of oxygen, causing iron oxide or corrosion. Excess oxygen will decrease the density of planets. The surface of Mars gets a red tint from iron oxide, but like its three terrestrial siblings, it has a core made of non-oxidized iron. Conversely, if the lower density of planetary TRAPPIST-1 is due to fully oxidized iron, the planets will have to be completely rusted and cannot have iron cores.
Agol said the answer may be a combination of two scenarios – less iron overall and some oxidized iron.
An artistic depiction of three possible interiors of TRAPPIST-1 exoplanets. The more accurate scientists know the density of a planet, the more they can reduce the range of potential intrinsic to that planet. All seven planets have very similar densities, so they are likely identical compositions.NASA / JPL-Caltech
The team also looked at whether the surface of each planet could be covered with water, which is lighter than corrosion and which would change the overall density of the planet. If this were the case, water would account for about 5% of the total mass of the outer four planets. In comparison, water makes up less than 0.1% of the Earth’s total mass. The three internal TRAPPIST-1 planets, which remain very close to their star for water, require a warm, dense atmosphere on Venus to remain liquid under most circumstances, where water can remain bound to the planet in the form of steam is. But this explanation seems unlikely because it would be a coincidence for all seven planets that according to Egol, just enough water exists of such a similar density.
When it launches, NASA’s James Webb Space Telescope should have the ability to further investigate the system, including collecting more detailed information about the seven TRAPPIST-1 world’s atmospheres.
“There are many more questions about Trappist-1 and its world,” Agol said. “And in a way, answering them helps us understand our solar system as well.”
Agol and Meadows are members of the NASA NExSS Virtual Planetary Laboratory team and the UW Astrobiology Program. Agol’s participation in the study was funded by the National Science Foundation, NASA, the Guggenheim Foundation and the Virtual Planetary Laboratory.
For more information, contact Agol [email protected].
Adapted from a story By NASA’s Jet Propulsion Laboratory.
Tag (s): Astronomy and Astrophysics • College of Arts and Sciences • Department of Astronomy • Eric Agole – Planetary Sciences