A new study of the tightly-packed family of planets forty light years from Earth has confirmed that it might be the simplest place to seek out life outside our solar system.The ‘holy grail’ Trappist-1 planetary system hit the headlines a year ago when astronomers revealed that it contained a multitude of Earth-like worlds. Since then, the Hubble Space Telescope has been used to capture faint tell-tale light signals as the planets passed in front of their star.
- Researchers conducted four studies which refined their knowledge of the star of the centre of the system
- The researchers found that all seven planets are mostly made of rock, with up to 5% of their mass in water
- By contrast, Earth’s oceans only account for 0.02% of its mass, meaning these planets hold a lot of water
- The researchers also found that the seven planets are considered temperate, meaning that under certain geological and atmospheric conditions, all could have conditions that allow water to remain in liquid form
In addition, five of the planets appear devoid of an atmosphere made of Hydrogen and Helium, like for Neptune or Uranus. This new information reinforces the notion that the seven planets of TRAPPIST-1 are similar to the rocky worlds of the Solar system in many ways.
The form that water takes on TRAPPIST-1 planets would depend on the amount of heat they receive from their star, which is a mere 9 percent as heavy as our Sun.The seven planets are considered temperate, meaning that under certain geological and atmospheric conditions, all could possess conditions allowing water to remain liquid. Work, including the team’s series of results, is now proceeding to pinpoint which of these temperate planets are most likely to be habitable.
Astronomers used Hubble to analyze light from the nearby star TRAPPIST-1 that passed through the atmospheres of four Earth-sized planets in the star’s habitable zone. Top: A model spectrum containing the signatures of gases the astronomers would expect to see if the exoplanets’ atmospheres were puffy and dominated by primordial hydrogen from the distant worlds’ formation. Bottom: the flat spectrum shown indicates that Hubble did not spot any traces of water or methane.
Professor Brice-Olivier Demory, co-author at the University of Bern, added, “Densities, while important clues to the planets’ compositions, do not say anything about habitability. However, our study is an important step forward as we continue to explore whether these planets could support life.”As part of this series of work, the team used the Hubble Space Telescope while the planets passed in front of their star, attempting to catch minute signals while starlight interacts with the planets’ atmospheres.
WHAT IS THIS TRAPPIST-1 SYSTEM?
TRAPPIST-1 is a planetary system consisting of seven planets orbiting an ultra-cool dwarf star, called TRAPPIST-1, about 40 light-years away in the Aquarius constellation.
TRAPPIST-1 is named after the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, which discovered two of the seven planets we know of today – announced in 2016.
NASA’s Spitzer Space Telescope, in collaboration with ground-based telescopes, confirmed these planets and uncovered the other five in the system.
The TRAPPIST-1 planets huddle so close to one another that a person standing on the surface of one of these worlds would have a spectacular view of the neighbouring planets in the sky, which would sometimes appear larger than the Moon looks to an observer on Earth.
What might these planets be like?
It is impossible to know exactly how each planet looks, because they are so far away. In our own solar system, the Moon and Mars have nearly the same density, yet their surfaces appear entirely different.The Trappist-1 planets circle a cool red-dwarf star just 9 per cent as massive as the sun.Because it is so faint, the star’s ‘habitable zone’ – the orbital region where water can exist as a liquid – is much closer in than the sun’s.
As a result the seven planets are all nearer their star than Mercury is to the sun, yet enjoy relatively mild climates.The worlds are also so huddled together that a person standing on any one of them would have a spectacular view of their celestial neighbours.In some cases, the planets could appear larger than the moon seen from the Earth, said the astronomers.
Based on available data, here are scientists’ best guesses about the appearances of the planets:
This illustration shows the seven Earth-size planets of TRAPPIST-1. The image does not show the planets’ orbits to scale, but highlights possibilities for how the surfaces of these intriguing worlds might look
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, meanwhile, is the lightest of the planets—about 30 percent the mass of Earth. Scientists are uncertain whether it has a large atmosphere, an ocean or an ice layer—all three of these would give the planet an “envelope” of volatile substances that would make sense for a planet of its density.
Scientists were surprised that TRAPPIST-1e is the only planet in the system slightly denser than Earth, suggesting it may have a denser iron core than our home planet. Like TRAPPIST-1c, it does not necessarily have a thick atmosphere, ocean or ice layer—making these two planets distinct in the system. It is mysterious why TRAPPIST-1e is so much rockier in its 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 most similar planet to Earth.
TRAPPIST-1f, g and h are far enough from the host star that water could be frozen as ice across these surfaces. If they have thin atmospheres, they would be unlikely to contain the heavy molecules of Earth such as carbon dioxide.
How do we know?
Scientists are able to calculate the densities of the planets because they happen to be lined up such that when they pass in front of their star, our Earth and space-based telescopes detect a dimming of its light. This is called a transit. The amount by which the starlight dims is related to the radius of the planet.
‘Humans definitely need oxygen to live, but we need oxygen at a certain concentration,’ said Dr Triaud. ‘For instance it is harder to breath atop mountains.’When asked whether humans could live in the system if we built support structures, Dr Triaud said it may be possible. ‘In the ALMA observatory, in Chile, the buildings compensate for the lack of oxygen concentration,’ he said. ‘Another point to consider is the atmospheric pressure (oxygen or no).
‘Too much pressure and we would need a space suit, too little (like on Mars, or like in space where pressure is 0) and we also need space suits. To get the density, scientists take advantage of what is called “transit timing variations.”
If there were no other gravitational forces on a transiting planet, it would always cross in front of its host star in the same amount of time—for example, Earth orbits the Sun every 365 days, which is how we define one year. But because the TRAPPIST-1 planets are packed so close together, they change the timing of each other’s “years” ever so slightly. Those variations in orbital timing are used to estimate the planets’ masses. Then, mass and radius are used to calculate density.
As a next step, the researchers plan to use NASA’s James Webb Space Telescope, which will be able to help answer the question of whether these planets have atmosphere and, if so, what those atmospheres consist of and whether they allow adequate surface conditions for liquid water to formThe next step in exploring TRAPPIST-1 will be NASA’s James Webb Space Telescope, which will be able to delve into the question of whether these planets have atmospheres and, if so, what those atmospheres are like, and whether they allow adequate surface conditions to permit liquid water.
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