Scientists are BAFFLED after discovering a giant planet orbiting a tiny star
Astronomers have discovered a strange giant planet orbiting a tiny star - and nobody knows how it got there.
The star is a distant red dwarf known as TOI-6894 which is just a fifth the mass of our own sun and shines 250 times more faintly.
According to all known theories of planetary formation, a planet this small should only be home to small planets the size of Earth or Mars.
However, scientists have been baffled to discover a massive gas giant slightly larger than Saturn orbiting this tiny sun.
Located about 240 light-years from Earth in the constellation Leo, TOI-6894 is now the smallest known star to host a large orbiting planet.
In a paper published in Nature Astronomy, the researchers reveal that this tiny star is a full 60 per cent smaller than the previous record holder.
Although the orbiting gas giant isn't quite larger than its star, they are much closer in size than should normally be possible.
Co-author Dr Vincent Van Eylen, an astronomer from UCL, says: 'It’s an intriguing discovery. We don’t really understand how a star with so little mass can form such a massive planet!'
Scientists have discovered a massive gas giant orbiting a tiny red dwarf star, and nobody currently knows how it got there (artist's impression)
The star TOI-6894 (pictured) is about 240 light-years from Earth in the constellation Leo. It is the smallest star to host a large planet in its orbit (circled in red)
In our solar system, the sun is 10 times the size of Jupiter, the largest planet in orbit.
By contrast, TOI-6894 is just 2.5 times larger than its only orbiting planet, known as TOI-6894b.
The planet is slightly larger than Saturn and a bit smaller than Jupiter, but far less dense than either.
Despite its size, the planet's mass is only 56 per cent that of Saturn and just 17 per cent that of Jupiter.
However, this isn't the only unusual feature of this cosmically mismatched pair.
Due to this planet's size, TOI-6894b sits 40 times closer to its star than Earth does to the sun and completes an entire orbit in just three days.
That proximity makes the planet much hotter than Earth - and not a good candidate for alien life - but it is nowhere near as hot as scientists would expect.
Most exoplanets spotted by astronomers are what scientists call 'hot Jupiters'.
In our solar system (artist's impression) the sun is 10 times the diameter of the largest planet, Jupiter. TOI-6894b is only 2.5 times smaller than its star, something scientists previously thought was impossible
These graphs show how the planet TOI-6894b compares to other known exoplanets. These show that the planet is much closer in size to its host star than most planets
These are massive gas giants with temperatures exceeding 1,700°C (3,100°F).
However, since the red dwarf sun is so cool the planet's atmosphere is just 147°C (300°F) which has big implications for its chemical makeup.
Professor Amaury Triaud, of the University of Birmingham, says: 'Based on the stellar irradiation of TOI-6894b, we expect the atmosphere is dominated by methane chemistry, which is very rare to identify.
'Temperatures are low enough that atmospheric observations could even show us ammonia, which would be the first time it is found in an exoplanet atmosphere.'
But the strangest thing of all about this distant planet is that, according to our best theories, it shouldn't exist at all.
Scientists' best explanation for how planets form is called the core accretion theory.
The birth of a planetary system begins with a large cloud of gas and dust - called a molecular cloud - that collapses under its own gravity to form a central star.
Leftover material spinning around the star in what is called a protoplanetary disc forms planets as the material clumps together under its own gravity.
The best theory for how planets form is the accretion theory. This suggests that planets form out of the accretion disk left behind from the birth of a young star
In the case of gas giants like Saturn or Jupiter, they first form a heavy core which pulls in and traps gas with its gravitational pull.
Small stars leave small protoplanetary discs which shouldn't contain enough material for a massive gas giant core to form.
However, the presence of a Saturn-sized planet orbiting this tiny red dwarf now suggests that this theory can't be completely accurate.
Lead author Dr Edward Briant, who completed the work at both UCL’s Mullard Space Science Laboratory and the University of Warwick, says there are two possible explanations.
The planet may have formed through an 'intermediate core-accretion process' in which a protoplanet forms and slowly gathers gas without becoming big enough to trigger the normal runaway gas accretion.
'Alternatively, it could have formed because of a gravitationally unstable disc,' says Dr Briant.
'In some cases, the disc surrounding the star will become unstable due to the gravitational force it exerts on itself.
'These discs can then fragment, with the gas and dust collapsing to form a planet.'
Small stars like red dwarfs have very small protoplanetary disks (artist's impression) which should allow for massive gas giants to form. The discovery of TOI-6894b means that our best theories about planetary formation cannot be entirely correct
Since red dwarf stars are extremely common in the universe, revealing how TOI-6894b formed could have big consequences for our search for exoplanets.
Co-author Dr Daniel Bayliss, of the University of Warwick, says: 'Most stars in our galaxy are actually small stars exactly like this, with low masses and previously thought to not be able to host gas giant planets.
'So, the fact that this star hosts a giant planet has big implications for the total number of giant planets we estimate exist in our galaxy.'
The atmosphere of TOI-6894b is due to be observed by the James Webb Space Telescope within the next 12 months.
By measuring the distribution of material within the planet astronomers will be able to work out the size and structure of the core.
This should allow scientists to determine which, if any, of these theories is correct.
However, until then, how this tiny star came to have such a large planet in its orbit will remain a perplexing mystery.
Scientists study the atmosphere of distant exoplanets using enormous space satellites like Hubble
Distant stars and their orbiting planets often have conditions unlike anything we see in our atmosphere.
To understand these new world's, and what they are made of, scientists need to be able to detect what their atmospheres consist of.
They often do this by using a telescope similar to Nasa's Hubble Telescope.
These enormous satellites scan the sky and lock on to exoplanets that Nasa think may be of interest.
Here, the sensors on board perform different forms of analysis.
One of the most important and useful is called absorption spectroscopy.
This form of analysis measures the light that is coming out of a planet's atmosphere.
Every gas absorbs a slightly different wavelength of light, and when this happens a black line appears on a complete spectrum.
These lines correspond to a very specific molecule, which indicates it's presence on the planet.
They are often called Fraunhofer lines after the German astronomer and physicist that first discovered them in 1814.
By combining all the different wavelengths of lights, scientists can determine all the chemicals that make up the atmosphere of a planet.
The key is that what is missing, provides the clues to find out what is present.
It is vitally important that this is done by space telescopes, as the atmosphere of Earth would then interfere.
Absorption from chemicals in our atmosphere would skew the sample, which is why it is important to study the light before it has had chance to reach Earth.
This is often used to look for helium, sodium and even oxygen in alien atmospheres.
This diagram shows how light passing from a star and through the atmosphere of an exoplanet produces Fraunhofer lines indicating the presence of key compounds such as sodium or helium
