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The Milky Way was recently believed to have a mass three trillion times bigger than that of our sun.

But now scientists have discovered that our Milky Way is lighter than previously thought - with just 800 billion solar masses.

This means it has about half the mass of one of its closest neighbours, the Andromeda galaxy, 2.5 million light years away.

The Milky Way is seen here above the dome of the Danish 1.54-metre telescope at La Silla Observatory in Chile. Scientists have discovered that the Milky Way, the galaxy containing the sun and its family of planets, is lighter than was previously thought

Both are similar types of spiral galaxy and the largest in a clump of 54 galaxies known as the 'Local Group'.

Scientists believe Andromeda's extra weight must be in the form of dark matter, an invisible substance thought to bind galaxies together with its gravity.

The galaxy, which is about the same size as the Milky Way, is believed to contain twice as much dark matter as our galactic home.

Previous studies were only able to measure the mass enclosed within the galaxies' inner regions. The new study included the mass of outer region invisible matter.

Scientists believe Andromeda's extra weight must be in the form of dark matter, an invisible substance thought to bind galaxies together with its gravity. At approximately 2.5 million light-years away, the Andromeda galaxy, or M31, is our Milky Way's largest galactic neighbour

Data on the known distances between galaxies in the Local Group were used to calculate the total masses of Andromeda and the Milky Way.

The findings, from a team led by Dr Jorge Penarrubia at the University of Edinburgh, appear in the journal Monthly Notices of the Royal Astronomical Society.

Dr Penarrubia said: 'We always suspected that Andromeda is more massive than the Milky Way, but weighting both galaxies simultaneously proved to be extremely challenging.

They researchers believe their work should help them learn more about how the outer regions of galaxies are structured. 

'Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible.' 

Although both galaxies appear to be of similar dimensions, until now scientists had been unable to prove which is larger. 

Dr Penarrubia told MailOnline: 'We don't understand at all what dark matter might be and there are many theories.

'What are research could do is help explain more about how this elusive material behaves.'

The Milky Way is thought to be 120,000 light years across and contains more than 200 billion stars. Here it is pictured above Steephill Cove in Isle of Wight, England