Science objectives

Charting the history of the Milky Way
Our Galaxy is a disc of some one hundred billion stars in a spiral structure surrounding a central bulge. While many stars were born and grew up here, many others originated in small external galaxies that have subsequently merged with ours.

Since each celestial object preserves something of the era during which it was born, Gaia’s detailed census of every star’s position, motion, brightness and colour allows astronomers to painstakingly piece together the history of our Galaxy.

Gaia can discriminate between generations of star – essentially plotting a ‘family tree’ of the Milky Way’s history. As stars condense out of celestial clouds of gas, they naturally incorporate the chemicals found in space. The stars then process this material in their hearts and expel it at the end of their lives, enriching the Galaxy with new, heavier chemical elements that are incorporated into the next generation of stars.

Gaia is also able to detect the motions of the stars as they move through space. Taken with the compositions, Gaia allows astronomers to identify families of stars that may have once belonged to different galaxies outside of the Milky Way, but which were since consumed by our Galaxy. Thus Gaia essentially puts the motions of the Milky Way’s stars into ‘rewind’ to learn more about where they came from.

Alien worlds
Gaia is looking at hundreds of thousands of stars beyond our own Solar System to about 500 light years from Earth. It is watching for tiny wobbles as they are tugged by the gravitational pull of orbiting planets. Estimates suggest that Gaia will detect thousands – if not tens of thousands – of Jupiter-sized planets beyond our Solar System throughout the course of its mission.

Rogue asteroids
Detecting asteroids and assessing their threat to Earth commands a great public and scientific interest. Gaia is contributing to this search through its unprecedented sensitivity to faint, moving objects. It is expected to detect tens of thousands of asteroids, some of which will be near-Earth objects, while others will live in the 'main-belt' of asteroids between Mars and Jupiter, or in the icy realms of the outer Solar System known as the Kuiper Belt.

Gaia is also ideally suited to search for asteroids close to the Sun – a blind spot where ground-based telescopes cannot see because they would have to look into the daytime sky. It is able to study their surface properties and rotation rates by measuring how much light is reflected in a particular colour.

Gaia also studies the asteroids we already know to exist, accurately determining the positions and velocities of more than 200,000 asteroids in order to refine their orbits with unprecedented precision.

Comet showers
A large cloud of icy bodies – comets – envelops the Solar System. Occasionally comets are dislodged by a passing star, sending a shower of icy debris into the inner reaches of the Solar System, peppering the Earth and other planets. As Gaia surveys the stars, it is able to rewind their motions to reveal those stars that may be the perpetrators of such comet showers. Equally, Gaia’s census will help predict future encounters.

Failed stars
Gaia is on the hunt for brown dwarfs – ‘failed stars’ that fell short of the mass needed to spark nuclear fusion in their hearts to achieve star-status. Vast numbers are predicted to be drifting through space, but observations appear to be falling short. Dubbed ‘the brown dwarf desert’, Gaia will track down these ‘missing’ objects – if they are there.

Exploding stars
One possible end point for a star’s life is to burn out in a dramatic supernova explosion, briefly lighting the sky with a flood of radiation. Over the course of its mission, Gaia is expected to detect tens of thousands of supernova in distant galaxies, before they reach their maximum brightness. Often these events are not noticed until during or even after their maximum brightness, but it is essential to observe such supernovae as they are still climbing in brightness because the peak brightness is the essential measurement for calculating their distance. Gaia is able to provide an early warning system for astronomers on Earth who wish to study supernovae as they reach their maximum brightness.

Space warps
Space isn’t flat; it contains numerous warps and dips caused by the gravity of massive objects like the Sun and planets. These ‘dips in the road’ can deflect light from a straight line of travel. Gaia’s sensitivity allows astronomers to see this deflection with the highest precision ever. Albert Einstein’s General Theory of Relativity describes these warps and how they must be taken into account when interpreting data such as from Gaia. In turn, Gaia’s measurements offer scientists the chance to test key parts of Einstein’s equations to unprecedented levels. It will provide an upper limit on the strength of gravitational waves – ripples in the fabric of space-time, and confirm the strength of gravity.