At least two planets that could be bigger than Earth might have been hiding at the edge of our solar system, scientists have claimed.

The undiscovered planets are thought to be even further from the Sun than Neptune and the dwarf planet Pluto.

Evidence of the two planets comes from watching a belt of space rocks known as “extreme trans-Neptunion objects”.

Scientists would normally expect the rocks to be randomly distributed. But they’re moving in completely unexpected ways, which seem to indicate that they’re being pulled by something that can’t be seen.

Spanish lead scientist Professor Carlos de la Fuente Marcos, from the Complutense University of Madrid (UCM), quoted by the Spanish scientific news service (Sinc), said: "This excess of objects with unexpected orbital parameters makes us believe that some invisible forces are altering the distribution of the orbital elements of the Etno, and we consider that the most probable explanation is that other unknown planets exist beyond Neptune and Pluto.

Astronomers have spent decades debating whether a hidden planet beyond Pluto remained undiscovered.

The new research, published in the journal Monthly Notices of the Royal Astronomical Society Letters, is based on analysis of an effect called the "Kozai mechanism", by which a large body disturbs the orbit of a smaller and more distant object.

Space in perspective: this is how big the Earth really is compared to the other planets in our Solar System Show all 7 1 /7 Space in perspective: this is how big the Earth really is compared to the other planets in our Solar System Space in perspective: this is how big the Earth really is compared to the other planets in our Solar System Saturn "Jupiter holds the title for being the biggest planet in the solar system, but Saturn is no tiddler," says amateur astronomer John Brady, who made these composite images. Like shoulder pads (or a massive hat) those rings let it dress up and seem even bigger than it is - but you could still fit some 764 Earths inside the planet itself. Space in perspective: this is how big the Earth really is compared to the other planets in our Solar System Saturn's Rings The rings themselves are made up of billions of bits of ice and rock debris, ranging in size from tiny grains to mountain-sized chunks. There's a number of gaps in the rings, some of which are patrolled by moons sweep a clear path through the junk like bodyguards, while others -like the largest gap, the Cassini Division - are caused by gravitional resonances between debris and those same moons. Space in perspective: this is how big the Earth really is compared to the other planets in our Solar System Neutron Star There isn't actually a Neutron Star in our Solar System - but we should be thankful for that. It's only 20km across (shown here sitting on the North West coast of England) but has 1.5 times the mass of our Sun - just a teaspoon of the material would weigh more than a billion tonnes. They also spin incredibly fast - faster than a kitchen blender, rotating as quickly as 716 times a second. Space in perspective: this is how big the Earth really is compared to the other planets in our Solar System The Sun Here comes the Sun: it makes up 99.86 per cent of the Solar System's entire mass and puts out more energy in a single second than humans have ever produced in our entire history. All the planets are in fact just byproducts from the Sun's formation and when it goes, we go with it - though that won't be for another 7.6 billion years. Space in perspective: this is how big the Earth really is compared to the other planets in our Solar System Olympus Mons Some of Mars' geographical features dwarf those of Earth. Olympus Mons isn't just the biggest volcano on the planet, it's bigger than any in the Solar System and in this image dwarfs the state of Arizona. Planetary geologists think Mons' massive size is to do with the lack of active plate tectonics on Mars. This means that magma can continually erupt in the same spot without moving. It's so big that it's tough to know how to measure it, but it's generally agreed it's at least 22km high - nearly three times taller than Mount Everest. Space in perspective: this is how big the Earth really is compared to the other planets in our Solar System Mars And here's Mars itself against the same portion of North America. As Brady points out, this shows that you could fly half way around Mars in around eight hours - though of course you'd need an incredibly light plane as Mars' atmospher is only 1 per cent as thick as Earth's. "Earth’s rusty neighbour in the solar system is the second smallest of the planets, Mercury being smallest," says Brady. "The actual dry land mass of Mars is around the same as Earth’s, because although Mars is much smaller it doesn’t of course have any seas." Space in perspective: this is how big the Earth really is compared to the other planets in our Solar System Jupiter Canada and the US combined are dwarfed on the surface of Jupiter - the biggest planet in the solar system, with a total mass greater than all the other planets and moons put together. "Jupiter is the king of the solar system," says Brady. "It's over 11 times the diameter of our planet, with lightning bolts up to 1,000 times more powerful than Earth’s, and wind speeds in the upper atmosphere that can reach 100 metres per second. This planet races around in just 10 hours compared to Earth’s 24, making it the fastest rotating planet in the solar system."

The scientists wrote: "In this scenario, a population of stable asteroids may be shepherded by a distant, undiscovered planet larger than the Earth ... "

One problem is that the theory goes against predictions of computer simulations of the formation of the Solar System, which state there are no other planets moving in circular orbits beyond Neptune.

But the recent discovery of a planet-forming disk of dust and gas more than 100 astronomical units (AU) from the star HL Tauri suggests planets can form long distances away from the centre of a solar system.

An astronomical unit, the distance between the Earth and the Sun, is the equivalent of 93 million miles.

"The exact number is uncertain, given that the data that we have is limited, but our calculations suggest that there are at least two planets, and probably more, within the confines of our Solar System."

More results based on a larger sample of Etno objects are due to be published in the coming months.

"If it is confirmed, our results may be truly revolutionary for astronomy," said Prof De La Fuente Marcos.