Nasa has found three new planets – including a kind of world unseen in our own solar system.

The mysterious planets, part of the TOI-270 system, are ”missing link” worlds and could be a huge gift to researchers looking for alien worlds, they said.

The three planets orbit a star that is only 73 light years away. It makes them among the closest exoplanets ever found, as well as being among the smallest.

They were discovered by researchers using Nasa’s Exoplanet Survey Satellite (TESS), which was shot into space in 2018 and has been scanning the universe for stars and planets that could support alien life.

TOI-270 has a rocky super-Earth, which is slightly bigger than our planet, and two gaseous planets that are lightly larger. That makes them a “missing link” – sitting between the smaller rocky worlds like our own Earth or Mars, and the much larger gaseous planets such as Saturn and Jupiter.

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Researchers hope to use the solar system to understand why there are so few worlds of that size, as well as helping how the planets in our solar system were found.

“TOI-270 will soon allow us to study this ‘missing link’ between rocky Earth-like planets and gas-dominant mini-Neptunes, because here all of these types formed in the same system,” said lead researcher Maximilian Gunther, from Massachusetts Institute of Technology.

Alongside the Earth-like exoplanet are two gaseous worlds, just over twice the size of our own. One of those, the furthest from the star, is thought to sit in the temperature range that could allow it to support alien life – but its atmosphere is expected to be so thick and dense that it stores lots of heat, and the surface is probably too warm.

Scientists hope to learn more about the relatively nearby worlds. As well as being close by and playing host to such unusual worlds, the star is unusually bright, without the solar flares and storms that can sometimes get in the way of observations.

“TOI-270 is a true Disneyland for exoplanet science, and one of the prime systems TESS was set out to discover,” Mr Gunther continued.

The unusual solar system – which might hold yet more planets (Nasa/Goddard Space Flight Centre/Scott Wiessinger) (NASA's Goddard Space Flight Center/Scott Wiessinger)

“It is an exceptional laboratory for not one, but many reasons – it really ticks all the boxes.”

There might be yet more planets in this solar system waiting to be found, researchers said.

While the planets are interesting enough in themselves, they also link together in a “resonant chain”, according to the researchers who found them. That means that their orbits are lined up neatly in whole integers, meaning that they are in “resonance” with each other.

“For TOI-270, these planets line up like pearls on a string,” Mr Gunther said. “That’s a very interesting thing, because it lets us study their dynamical behaviour. And you can almost expect, if there are more planets, the next one would be somewhere further out, at another integer ratio.”