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A spacecraft called Gaia launched by Esa to map the stars in the Milky Way could be repurposed to find planets in the galaxy, say scientists.

They say it could be used to find 70,000 worlds outside the solar system, including many that have previously been difficult to find.

This would include huge Jupiter-sized worlds around dim stars - but might not include Earth-like habitable worlds. 

Researchers from Princeton University in New Jersey and Lund University in Sweden say Gaia (illustration shown) could find thousands of planets during its mission. Primary goal of the spacecraft is to map one billion stars in the Milky Way. But a 'by-product' could also be the discovery of planets

Researchers from Princeton University in New Jersey and Lund University in Sweden calculated that the observational satellite Gaia could detect as many as 21,000 exoplanets during its five-year mission

If extended to 10 years, Gaia could detect as many as 70,000 exoplanets, the researchers report.

Exoplanets will be an important 'by-product' of Gaia's mission, first author Dr Michael Perryman of Princeton University said.

Built and operated by Esa and launched in December 2013, Gaia will capture the motion, physical characteristics and distance from Earth - and one another - of roughly one billion objects, mostly stars, in the Milky Way galaxy with unprecedented precision.

The presence of an exoplanet will be determined by how its star 'wobbles' as a result of the planet's orbit around it.

More important than the numbers of predicted discoveries are the kinds of planets that the researchers expect Gaia to detect, many of which - such as planets with multi-year orbits that pass directly, or transit, in front of their star as seen from Earth - are currently difficult to find.

The satellite's instruments could reveal objects that are considered rare in the Milky Way, such as an estimated 25 to 50 Jupiter-sized planets that orbit faint, low-mass stars known as red dwarfs.

Unique planets and systems - such as planets that orbit in the opposite direction of their companions - can inspire years of research according to Dr Perryman.

'It's not just about the numbers. Each of these planets will be conveying some very specific details, and many will be highly interesting in their own way,' Dr Perryman said.

One of the main objectives of the Gaia mission is to establish the currently uncertain distance from Earth to various stars, which would allow a much better understanding of the properties of the stars and the planets orbiting them. This image shows the distances from Earth (centre) to the stars (black dots) of some exoplanets

The kinds of planets that the researchers expect Gaia to detect would include planets with multi-year orbits and others that are more difficult to detect via other methods. This would include Jupiter-sized worlds orbiting dim stars, but is unlikely to include many Earth-like planets (Earth-like Kepler 186f illustration shown)

'If you look at the planets that have been discovered until now, they occupy very specific regions of discovery space.

'Gaia will not only discover a whole list of planets, but in an area that has not been thoroughly explored so far.'

Ultimately, a comprehensive census allows scientists to more accurately determine how many planets and planetary systems exist, the detailed properties of those planets, and how they are positioned throughout the galaxy.  

'Our assessment will help prepare exoplanet researchers for what to expect from Gaia,' Dr Perryman added.

'We're going to be adding potentially 20,000 new planets in a completely new area of discovery space. It's anyone's guess how the field will develop as a result.'

Princeton University and Lund University researchers project that the recently launched European satellite Gaia could discover tens of thousands of planets during its five-year mission. In this image, the colored portions indicate the number of observations Gaia would make of a particular part of the sky during its mission