Previewing a scenic space selfie: This groundbreaking spacecraft crafted an unprecedentedly intricate image of our celestial Milky Way.
In a historic milestone, the Gaia space telescope concluded its 11-year mission on January 15, 2025, by surveying the Milky Way, delivering the most comprehensive and precise 3D map of our galaxy to date. This stellar census, encompassing motions, luminosities, temperatures, and compositions, has revolutionized our understanding of the cosmos.
Launched by the European Space Agency on December 19, 2013, Gaia traveled 930,000 miles (1.5 million km) to a gravitationally stable location known as Lagrange point 2. From this vantage point, it faithfully orbited the Sun, mirroring the James Webb Space Telescope's position since its arrival in 2021. Despite enduring solar radiation and micrometeorite bombardment, Gaia amassed more than 3 trillion observations of 2 billion stars and other objects during its mission.
Measuring the Galaxy's Fabric
Gaia's mapping method differed from previous approaches by leveraging parallax, the apparent back-and-forth movement of stars caused by Earth's changing perspective as it orbits the Sun. Its two optical telescopes, equipped with three science instruments measuring position, velocity, and brightness, repeatedly captured the positions of objects up to 400,000 times fainter than the human eye can perceive.
The result was a 200-fold enhancement in the accuracy of stellar positions and motions compared to the Hipparcos space telescope's data. With its high-resolution CCD cameras and larger light-collecting surface, Gaia provided an unprecedented level of detail, revealing the Milky Way's complex structure and possible evolution.
Shedding Light on the Shape of the Milky Way
The Milky Way, familiar to many as a barred spiral galaxy with spiral arms and a diffuse halo, has been found to have a more elongated and tilted form, akin to a football or rugby ball. Previous assumptions about the halo's beachball shape and uniform density have been challenged as Gaia data suggests it stretches and is elongated due to an ongoing merger with the Sagittarius Dwarf Galaxy.
This merger, estimated to have occurred about 4-5 billion years ago, has contributed to the warping of the Milky Way's disc and caused multiple bursts of star formation, including the one that led to our Sun's birth.
Unearthing Cosmic History
The collision of the Sagittarius Dwarf Galaxy was not an isolated event. Gaia discovered a stream of approximately 30,000 stars that moved in an opposite direction to the majority of stars in our galaxy, traveling along elongated trajectories. These stars shared the chemical composition of a different galaxy and were collectively known as Gaia-Sausage-Enceladus, having merged with the Milky Way around 10 billion years ago.
A family tree of smaller galaxies, merged and interacted with our own over billions of years, leaving a rich legacy of countless variable stars, globular clusters, and stellar streams.
A Detailed Look at Our Galactic Neighborhood
By providing a wealth of information about the Milky Way's structure and evolution, Gaia offered insight into our local area as well. Gaia's data showed that our Sun traverses an almost empty, peanut-shaped space bubble known as the Local Bubble. This discovery suggests a past influence of exploding stars that led to the formation of nearby newborn stars in our galaxy.
Furthermore, the data revealed an ancient population of spinning filaments, remnants of 'fossilized' spiral arms spun up by satellites that once passed through the Milky Way.
Addressing Cosmic Conundrums
By tracking the large-scale motion of stars, Gaia enabled scientists to infer the distribution and gravitational effects of dark matter, the invisible substance thought to hold the galaxy together. Additionally, the Hubble Space Telescope's data, along with Gaia's increased accuracy, has resulted in a recalibration of the Universe's expansion rate.
However, discrepancies remain between the expansion rate of nearby and distant galaxies, reflecting a "tension" between the two measurements that remains unresolved. Meanwhile, Gaia has produced the most extensive 3D-map of around 1.3 million quasars, shed light on General Relativity, and allowed for the testing of the theory to new levels of precision.
Exploring the Solar System and Beyond
In addition to its groundbreaking contributions to understanding the Milky Way, Gaia provided valuable data on asteroids and exoplanets. The space telescope collected pinpoint positions and motions for over 150,000 asteroids, offering an opportunity for scientists to discover asteroids with companion moons. Furthermore, Gaia's inventory of asteroids' chemical makeup will enable researchers to identify their material composition.
As for exoplanets, Gaia detected thousands through changes in stellar positions due to orbiting planets or by observing a dip in brightness when a planet transits a star's face. This includes gas giants, brown dwarfs, and planets orbiting low-mass stars, which will aid scientists in understanding the characteristics of diverse planet types and their occurrence frequencies.
At the end of its mission, Gaia's nitrogen tank was emptied after several weeks of testing, and it is now being relocated to an orbit close to the Sun and out of Earth's sphere of influence. Although the telescope has closed its gaze on the cosmos, it will leave a lasting impact through two major data releases, one planned for mid-2026 and another scheduled for the end of the decade.
In summary, the Gaia space telescope has leveled a unique perspective on the Milky Way, offering a detailed understanding of our galaxy's birth, evolution, shape, and composition. Its contributions to astrophysics will continue to reverberate in the scientific community for years to come.
- The Gaia space telescope's 200-fold enhancement in the accuracy of stellar positions and motions, compared to the Hipparcos space telescope's data, transformed our understanding of the cosmos by revealing the Milky Way's complex structure and possible evolution.
- Contrasting previous assumptions about the Milky Way's halo, Gaia's data suggest that it stretches and is elongated due to an ongoing merger with the Sagittarius Dwarf Galaxy, giving the Milky Way a more elongated and tilted shape, similar to a football or rugby ball.
- Gaia's discovery of Gaia-Sausage-Enceladus, a stream of approximately 30,000 stars that share the chemical composition of a different galaxy, indicates a past merger of our galaxy with another around 10 billion years ago.
- Gaia's mapping of our Sun's local space revealed the existence of an almost empty, peanut-shaped space bubble known as the Local Bubble, suggesting a past influence of exploding stars that led to the formation of nearby newborn stars in our galaxy.
- Gaia's observations of stellar motions allowed scientists to infer the distribution and gravitational effects of dark matter, an invisible substance thought to hold the galaxy together.
- In addition to understanding the Milky Way, Gaia provided valuable data on asteroids and exoplanets, offering an opportunity for scientists to discover asteroids with companion moons and shedding light on the characteristics of diverse planet types and their occurrence frequencies.
- Contemporary environmental science, space-and-astronomy, and cosmology fields will continue to benefit from Gaia's data, with two major data releases planned, the first for mid-2026 and another scheduled for the end of the decade.
