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What is a terrestrial planet?
The artistic concept of what rocky and potentially habitable worlds might look like in other parts of our galaxy. Data collected by telescopes in space and on the ground suggests that small rocky planets are common. (Placing them so close together in a line is for illustrative purposes only.) Credit: NASA / JPL-Caltech / R. Male
Terrestrial planets are Earth-sized and smaller, made up of rock, silicate, water and / or carbon. To determine if any of these worlds have atmospheres, oceans, or other signs of habitability, further investigation is needed. The largest terrestrial exoplanets (those at least twice as massive as Earth) are classified as super-Earths.
Get to know some terrestrial planets
Get to know some terrestrial planets
TRAPPIST-1e, the fourth planet from the star, is in the habitable zone, which means that liquid water may exist on its surface.
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The third planet from its star is illustrated here with a narrow band of water between the warm side of the day and the frosty night side of the planet.
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In general, terrestrial planets have a mass composition dominated by rock or iron and a solid or liquid surface. These distant worlds may have gaseous atmospheres, but this is not a defining feature.
We have found rocky planets in the size range of the Earth, at just the right distance from their parent stars to house liquid water (this is known as the habitable zone). While these characteristics do not guarantee a habitable world – we cannot yet say whether these planets really have atmospheres or oceans – they can help point us in the right direction.
Main aspects
Future space telescopes will be able to analyze light from some of these planets, looking for water or a mixture of gases that resemble our atmosphere. We will gain a better understanding of surface temperatures. As we keep ticking off items on the habitability list, we will move closer and closer to finding a world that bears recognizable signs of life.
Seven terrestrial worlds of TRAPPIST-1
Seven terrestrial worlds of TRAPPIST-1
In 2017, NASA announced the discovery of Earth’s largest planets found in the habitable zone of a single star, called TRAPPIST-1. This system of seven rocky worlds – all with the potential for water on their surface – is an exciting discovery in the search for life on other worlds. Future study of this unique planetary system could reveal conditions suitable for life.
This graph shows, in the upper row, the artist’s concepts of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, rays, masses, densities and surface gravity compared to those of the Earth. Credit: NASA / JPL-Caltech
In February 2018, a closer look at the seven planets suggested that some may harbor far more water than Earth’s oceans, in the form of atmospheric water vapor for the planets closest to their star, liquid water for others, and ice for those. farther away. That research fixed the density of each planet more precisely, making TRAPPIST-1 the best known planetary system aside from our own.
It is impossible to know exactly what each planet looks like, because they are so distant and so faint compared to their host star. In our solar system, the Moon and Mars are nearly the same density, but their surfaces appear completely different.
This artist’s concept shows what the TRAPPIST-1 planets might look like, based on available data on the diameters, masses and distances of the planets to the host star, as of February 2018. Credit: NASA / JPL-Caltech
Based on the available data, here are scientists’ best guesses about the appearance of planets:
TRAPPIST-1b, the innermost planet, is likely to have a rocky core, surrounded by an atmosphere much thicker than Earth’s. TRAPPIST-1c also likely has a rocky interior, but with a thinner atmosphere than planet b. TRAPPIST-1d is the lightest of the planets: about 30 percent of the mass of the Earth. Scientists are unsure whether it has a large atmosphere, an ocean or a layer of ice: all three of these would give the planet a “shell” of volatile substances, which would make sense for a planet of its density (less than half that). of the Earth).
Scientists were surprised that TRAPPIST-1e is the only planet in the slightly denser system on Earth, suggesting it may have a denser iron core than our home planet. Like TRAPPIST-1c, it doesn’t necessarily have a dense atmosphere, ocean, or ice sheet, which makes these two distinct planets in the system. It is mysterious why TRAPPIST-1e has a much more rock composition than the rest of the planets. In terms of size, density and the amount of radiation it receives from its star, this is the closest planet to Earth.
TRAPPIST-1f, g and h are far enough away from the host star that the water in their possession could be frozen like ice on these surfaces. If they have thin atmospheres, they are unlikely to contain Earth’s heavy molecules, such as carbon dioxide.
How many terrestrial planets are there out there?
How many terrestrial planets are there out there?
The more we study, the more our understanding can change. An analysis of the findings from NASA’s Kepler Space Telescope indicated that 20 to 50 percent of the stars in the sky are likely to have small and potentially rocky planets in their habitable zones. The most recent data has shown that the number is likely to be lower, perhaps between 2 and 12.
At first glance, this might seem disappointing: fewer rocky and potentially habitable worlds among the thousands of exoplanets found so far. But that doesn’t change one of the most surprising conclusions after more than 20 years of observation: planets in the habitable zone are common.
Much more data is needed, including a better understanding of how a planet’s dimensions relate to its composition.
“We’re still trying to figure out how big a planet can be and still be rocky,” said Jessie Dotson, an astrophysicist at NASA’s Ames Research Center in California’s Silicon Valley. She is also the project scientist for Kepler’s extended mission, known as K2. The spacecraft was retired in 2018, but discoveries are still ongoing using its data.
A six-year research that looked at millions of stars found that there are likely to be at least 1,500 planets within just 50 light-years of Earth.
The findings are based on observations made over six years by the Probing Lensing Anomalies NETwork (PLANET) collaboration, founded in 1995. The study concludes that there are far more Earth-sized planets than Jupiter-sized bloated worlds. This is based on research showing that the number of planets increases for worlds of lower mass. A rough estimate from this survey would indicate the existence of over 10 billion terrestrial planets in our galaxy.
A strange gap in the size of the planets
A strange gap in the size of the planets
Scientists have noticed what appears to be a strange gap in the size of the planets. It was nicknamed the Fulton Gap, after Benjamin Fulton, lead author of an article describing it. Kepler’s data shows that planets of a certain size range – those between 1.5 and 2 times the size of Earth – are rare. It is possible that this gap represents a critical split in planet formation: planets that reach the larger end of the radius gap quickly attract a dense atmosphere of hydrogen and helium and swell into gas planets, while planets smaller than the gap they are not large enough to maintain such an atmosphere and remain mostly rocky. On the other hand, smaller planets orbiting close to their stars may be the cores of Neptune-like worlds that have had their atmospheres stripped away.
Explaining the Fulton gap will require a much better understanding of how solar systems are formed.
Explore planet types: gas giant, Neptune-like, super-terrestrial and terrestrial
Or move on to the building blocks of galaxies: the stars!
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