Like other large galaxies, the Milky Way was built up from mergers with many smaller objects. Probably the most significant was with a smaller galaxy known as Gaia-Enceladus, but the timing of this event has eluded astronomers. Now an unexpected source, NASA's Transiting Exoplanet Survey Satellite (TESS), has helped narrow things down, having expanded far beyond its original role of seeking planets around other stars.
Although TESS was designed to detect the slight fading of stars as planets pass before them, it has turned out to be a wondrously flexible instrument, yielding scientific bonanzas in unexpected places.
The latest example comes from a study of ν Indi, a star (barely) visible to the naked eye from the southern hemisphere. Although its chemical composition indicates ν Indi is native to the Milky Way, its path around the galaxy would be impossible without having experienced a major disruption. Astronomers have attributed its strange course to the merger with Gaia-Enceladus.
The arrival of so much material in the Milky Way had a big effect on stars that happened to be close by, ν Indi included. "Since the motion of ν Indi was affected by the Gaia-Enceladus collision, the collision must have happened once the star had formed,” said Professor Bill Chaplin of the University of Birmingham in a statement.
Stars experience natural oscillations, whose frequencies evolve with them. Measuring these stellar earthquakes can reveal a star's age but is difficult for ground-based telescopes to do precisely. For TESS, on the other hand, it's a snap. Based on TESS's oscillation measurements Chaplin and a large team of co-authors conclude in Nature Astronomy ν Indi is 15 percent less massive than the Sun and, more importantly, 11 billion years old.
That still allows Gaia-Enceladus the majority of the age of the universe in which it could have been swallowed by the Milky Way, but other evidence suggests a minimum of 9 billion years ago.
The findings are just one example of TESS's astonishing multi-tasking. Last week it was announced that TESS had found its first Earth-sized world in a location suitable for liquid water (and therefore conceivably life).
The same day it was announced a high school student interning at NASA had used TESS's data to discover its first Tatooine planet, that is one orbiting two stars. The planet is far too large, and far too close to its parents, to support life, but imagine the sunsets.
To complete a stunning trifecta, TESS also produced evidence the binary stars Alpha Draconis A and B eclipse each other as seen from Earth. Eclipsing binaries are very useful for astronomical research, but not all that rare. What makes this discovery remarkable is that it involves such a well-studied star. Alpha Draconis is so well known it has a nickname (Thuban) and a reference in Futurama. It was the pole star 4-6,000 years ago before precession brought it further south from our perspective. Such a prominent star was not expected to hide secrets like this. “The first question that comes to mind is ‘how did we miss this?’” Dr Angela Kochoska of Villanova University said when presenting the finding.