Hopes of finding life on planets in a solar system similar to our own, known as Trappist-1, have been dashed after scientists found they may hold too much water.
Finding water on a planet is a good sign in the search for life, but in the case of these ‘water worlds’ they could have so much that they lack the chemicals needed.
The Trappist-1 system hit the headlines a year ago as a ‘holy grail’, when astronomers revealed that it contained a multitude of Earth-like worlds.
Three of these are in the so-called ‘goldilocks’ habitable zone, the range of orbits around a star in which a planet can support liquid water.
Experts say the seven planets detected have very low densities for their mass due to the volume of water and ice they hold – equivalent to hundreds of Earth-oceans.
Scientists say some of the celestial bodies are made up of more than 50 per cent water, compared to Earth’s 0.02 per cent water by mass.
Hopes of finding alien life on the Trappist-1 planets (artist’s impression) have been dashed after scientists found they may have the water equivalent to hundreds of Earth-oceans – which would be far too much to sustain life
Trappist-1 is an ultra-cool red dwarf star that is slightly larger than the planet Jupiter and is located about 40 light-years from the sun.
All of the planets are Earth-sized and terrestrial, making them an ideal place to look for alien life.
To determine the composition of the planets surrounding the star, researchers from Arizona State University and Vanderbilt University used mineral calculators.
The software, called ExoPlex, meant they could combine all the available information about the mass and radius of the planets as well as their chemical makeup.
They found the relatively ‘dry’ inner planets (‘b’ and ‘c’) had around 15 per cent water by mass.
The outer planets (‘f’ and ‘g’) had more than 50 per cent water by mass.
‘We typically think having liquid water on a planet as a way to start life, since life, as we know it on Earth, is composed mostly of water and requires it to live,’ said Dr Natalie Hinkel of Vanderbilt University, who was involved in the study.
Three of the planets are in the so-called ‘goldilocks’ habitable zone. This graph shows the minimum starting distances of the ice-rich Trappist-1 planets from their star (horizontal axis) as a function of how quickly they formed after their host star was born (vertical axis)
Scientists say some of the planets have up to 50 per cent water by mass. This is a huge amount considering our own blue planet is only around 0.02 per cent water by mass. Pictured is an artist’s impression
Finding water is a good sign but in the case of these ‘water worlds’ (artist’s impression) they could have so much they lack necessary geochemicals to sustain life . This image highlights possibilities for how the surfaces of these intriguing worlds might look
Experts found the relatively ‘dry’ inner planets (‘b’ and ‘c’) had around 15 per cent water by mass. The outer planets (‘f’ and ‘g’) had more than 50 per cent water by mass (artist’s impression)
‘However, a planet that is a water world, or one that doesn’t have any surface above the water, does not have the important geochemical or elemental cycles that are absolutely necessary for life.
‘It’s a classic scenario of “too much of a good thing”.’
Researchers also found that the ice-rich Trappist-1 planets are much closer to their host star than the ice line.
The ‘ice line’ in any solar system is the distance from the star beyond which water exists as ice.
Researchers suspect Trappist-1e is most like Earth. It is slightly denser than Earth, suggesting it might have a denser core than our planet. Like Trappist-1c, it doesn’t necessarily have a thick atmosphere, ocean or ice layer (artist’s impression)
WHAT IS THE 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.
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
The researchers 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
They may also be tidally locked, meaning the same side of the planet is always facing the star, and each side is in perpetual day or night. Although the planets are all closer to their star than Mercury is to the Sun, TRAPPIST-1 is such a cool star that its planets are temperate.
Based on available data, here are scientists’ best guesses about the appearance of the planets:
- TRAPPIST-1b: The system’s 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: The lightest of the planets – about 30 per cent the mass of Earth. Researchers are not certain whether it has a large atmosphere, an ocean or an ice layer. All three of these would provide the planet an ‘envelope’ of volatile substances.
- TRAPPIST-1e: Researchers suspect that this planet is most like Earth. It is slightly denser than Earth, suggesting it might have a denser core than our planet. Like TRAPPIST-1c, it doesn’t necessarily have a thick atmosphere, ocean or ice layer – making these two planets distinct in the system. In addition, the planet is a lot rockier 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 to planet Earth.
- TRAPPIST-1f, g and h: 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.
Through their analyses, the team determined that the Trappist-1 planets must have formed much farther from their star, beyond the ice line, and migrated in to their current orbits.
Knowing which planets formed inside and outside of the ice line allowed the team to quantify for the first time how much migration took place.
Because stars like Trappist-1 are brightest right after they form and gradually dim thereafter, the ice line tends to move in over time, like the boundary between dry ground and snow-covered ground around a dying campfire on a snowy night.
According to the researchers, the form that water takes on Trappist-1 planets would depend on the amount of heat they receive from their star, which is only nine per cent as heavy as our sun
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
The exact distances the planets migrated inward depends on when they formed.
‘The earlier the planets formed, the farther away from the star they needed to have formed to have so much ice,’ said Steven Desch, an Arizona State University astrophysicist and contributing author.
‘But for reasonable assumptions about how long planets take to form, the Trappist-1 planets must have migrated inward from at least twice as far away as they are now.’
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.
WHAT IS THE GOLDILOCKS ZONE?
In astronomy and astrobiology, the habitable zone is the range of orbits around a star in which a planet can support liquid water.
This habitable zone is also known as the ‘Goldilocks’ zone, taken from the children’s fairy tale.
The temperature from the star needs to be ‘just right’ so that liquid water can exist on the surface.
The boundaries of the habitable zone are critical.
If a planet is too close to its star, it will experience a runaway greenhouse gas effect, like Venus.
But if it’s too far, any water will freeze, as is seen on Mars.
Since the concept was first presented in 1953, many stars have been shown to have a Goldilocks area, and some of them have one or several planets in this zone, like ‘Kepler-186f’, discovered in 2014.
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.
The planets may also be tidally locked, meaning the same face is always pointing towards the star.
The full findings of the study were published in the journal Nature Astronomy.
Through their analyses, the team determined that the Trappist-1 planets must have formed much farther from their star, beyond the ice line, and migrated in to their current orbits