Essays on Gaia

Each planet around a star pulls on the star as it orbits, affecting the motion of the star itself. Multiple planets pull by different amounts, and with different periods. The result is that the star moves along a complex path through space, each planetary system with its own distinct 'fingerprint'. Gaia is measuring these remarkable movements.

Gravitational microlensing is the bending of light due to a massive body. It occurs through the chance alignment of stars along the line-of-sight. The changing brightness of the background star gives a powerful technique for discovering exoplanets. For the first time, tiny shifts in the positions of the distorted images can also be detected by Gaia.

Gaia is more than six years into a possible 10-year data collection phase. Three data releases have been issued to date. Each successive catalogue supersedes the previous. Each comprises more observations, and an increased temporal baseline, and an improving accuracy of the astrometric parameters as a result.

Scientists are keen to emphasise that their contributions are essential for the advancement of human knowledge, and that they often have unexpected benefits much further down the line. Some economists are actually trying to quantify the returns on a country's investment made in the areas of fundamental or basic science.

Classical astrometry measures star positions in two angular coordinates. But star motions along the line-of-sight can't be determined in this way. Great efforts were made to measure radial velocities from the ground for Hipparcos. Gaia is in the process of tracking more than a million of these radial motions directly from space.

Gaia uses a combination of advanced optical imaging and powerful onboard processing to detect every star that enters its field of view, down to a million times fainter than observable with the human eye. Each of these more than two billion stars are then tracked with its high-precision measurement system as the satellite scans the sky.

The design of the Gaia mission targeted very specific measurement goals, which were formulated in terms of limiting magnitude and astrometric accuracy. Gaia's limiting magnitude of around 20–21 mag, for example, was a requirement resulting from an examination of what types of stars, in different regions of our Galaxy, should be measured.

ESA's precursor to Gaia, Hipparcos, observed more than 100,000 stars in the early 1990s. The observing programme had to be pre-planned, in an effort that occupied many scientists for several years. That experience assisted the development of the techniques that Gaia uses today to detect everything that it observes directly on-board.

By the second half of the twentieth century the steady advance in the accuracy of stellar positions from the ground was running into a number of insurmountable barriers. The biggest problem was the bending and twinkling effects of the atmosphere. The answer was to carry out these measurements from space.

The motions of the stars and planets perplexed the curious minds of the ancient world. Great advances in understanding our place in the Universe came in the Middle Ages, driven in part by the practical demands of navigation. Instrumental advances over the past few centuries have continued to drive scientific enquiry ever since.

The positions of stars in the sky vary minutely with time for a number of reasons. Accurately and repeatedly measuring them over months and years can discern tiny motions which prove central to understanding their nature, as well as providing a wealth of information on the structure and origin of our Galaxy and beyond.

The basis of astrometric measurements is the accurate measurement of tiny angles that divide up the sky. Dividing a circle, whether on paper or on an imaginary sweep of the celestial sky, is a task well-posed in principle. Practical techniques for doing so aside, it is only necessary to agree on the unit of subdivision.