The Milky Way, Revealed as Never Before.

From before dawn until sunset on April 25, fueled by coffee and pastries (followed by steak and champagne), dozens of astrophysicists took over the third floor of the Flatiron Institute in downtown New York City, poring over gigabytes of fresh data from a once-in-a-lifetime space probe destined to forever change our understanding of the cosmos. Most of the scientists gathered in a cramped conference room, communing over laptops displaying arcane astroglyphs, but others migrated to breakout sessions scattered throughout the floor—sprawling belly-down on carpets or scrawling equations on whiteboards, occasionally muttering curses at server time-outs or compiler crashes that stymied their efforts to be among the world’s first to make dazzling new discoveries.

The probe is the European Space Agency’s Gaia satellite, a $1 billion mission that launched in 2013 to map our galaxy with remarkable precision; the fresh data came as Gaia released its second dataset. Based on 22 months of observations, the new release catalogues the positions, motions, brightnesses and colors of an astonishing 1.3 billion stars—roughly 1 percent of the estimated 100 billion stars that make up the Milky Way. At their best, the spacecraft’s measurements are akin to Earthbound observers discerning the position of a penny on the surface of the moon.

“A curtain has opened, and the Milky Way is now revealed,” says Jackie Faherty, an astronomer at the American Museum of Natural History who arrived at the Flatiron shortly after 5am for the start of a three-day gathering at the Institute’s Center for Computational Astrophysics (CCA) to delve deep into Gaia’s enormous data dump. “Today felt like the start of a gigantic race that’s going to last the rest of my career—or, really, until I’m dead.” What’s happening here is unprecedented, she says. “Any scientific question you could have about the galaxy will be linked to this dataset.”

Faherty isn’t exaggerating. Gaia’s first dataset, released two years ago, contained just two million stars or so. Another release is planned for 2020, and the last after the end of Gaia’s mission sometime in the 2020s. In each iteration, its measurements will improve—particularly its data on the “proper motion” of stars (that is, how fast they appear to move across the sky) and their parallax, the apparent shift in their position when seen from opposite sides of Earth’s orbit around the sun, with the nearest stars seeming to shift most.

By tracing the motions and positions of such a huge number of stars, Gaia is not only revealing new details about our galaxy’s spiral arms and other structures—it is also opening the window on previously hidden eras of the Milky Way’s history, in a practice known as galactic archaeology. Thanks to Gaia, astronomers can now wind back the clock on our galaxy, tracking its structural and stellar evolution across its 13-billion-year history. Among other things, they will be able pinpoint more accurately than ever the nurseries where different types of stars were born—a technique that could even reveal the sun’s lost galactic sister stars, which have long since scattered from the place they all arose. Gaia’s state-of-the-art reckoning of stellar positions will also vastly improve the star catalogs used for navigating interplanetary spacecraft.

But that is just the beginning. Beyond its bulk measurements of a billion-plus stars, Gaia’s latest release also includes the line-of-sight velocities for a subset of 7 million stars within a few thousand light-years of Earth, revealing their full three-dimensional motion through the galaxy. In addition, it tracks the fluctuating brightnesses of half a million variable stars, which astronomers can use to calibrate the first steps of the “cosmic distance ladder” that allows them to measure all the way to the edge of the observable universe.

Closer to home, Gaia has pinned down the movements of about 14,000 asteroids with an accuracy hundreds of times greater than previous surveys, allowing astronomers to better trace their formation histories and predict their future locations. And finally, by gazing far out past the Milky Way, Gaia now refines the position and brightness data for more than 500,000 quasars—cosmic beacons powered by supermassive black holes feeding on gas and dust at the hearts of distant galaxies. This network of quasars provides a celestial reference frame to better situate our solar system and the Milky Way within the cosmic landscape.

And in a choice atypical of most space missions, Gaia’s scientists have no period of exclusive access to the spacecraft’s results, honor-bound to solely address issues of data quality and collection prior to the dataset’s release. Only now, as the data become available to everyone worldwide, are members of the Gaia team beginning their own analyses and contributing to a global frenzy of fresh scientific discovery.


“It’s anyone’s guess what we’ll find,” says New York University astrophysicist David Hogg, who organized the CCA event. “One thing to mention is that, thanks to observing time provided by Columbia University, we will actually have access to telescopes at the MDM Observatory [near Tucson, Arizona] for a few days after the release. This permits us—and anyone who makes requests of us—to observe stars that seem interesting because of Gaia’s new data.” Most of his activity on “zero day” for Gaia’s data release, Hogg says, revolves around plotting new figures from the data to see whether anything eye-catching pops out.

And in fact, just hours after the release (which occurred at 6am Eastern time), Faherty and her collaborator Jonathan Gagné, an astronomer at the Carnegie Institution, had already obtained a new result thanks to Gaia’s precision parallax measurements: A system that may be a Rosetta Stone in our understanding of brown dwarfs, the so-called “failed stars” that walk the line between big planets and small suns. The system, which lies roughly 130 light-years away, in Earth’s southern skies, includes a brown dwarf in a wide, multi-thousand-year orbit around a small red dwarf star. This pair belongs in turn to a larger group of stars that contains other brown dwarfs, some orbiting much closer to their stars and some floating freely through space. By comparing the three types of brown dwarfs, astronomers might decipher how these in-between objects form in the first place, Faherty and Gagné say.


One of the most hotly anticipated results from Gaia is its potential haul of exoplanets. The spacecraft is expected to eventually reveal thousands of gas-giant planets by seeing their telltale gravitational influence on the positions and motions of their host stars (untold numbers of new binary-star systems will be found through similar observations).

But such discoveries will have to wait for the future, when Gaia’s readings attain even higher precision. For now, most of the mission’s relevance for exoplanets comes from its synergy with NASA’s Kepler space telescope, which discovered thousands of planets in a single starry patch of sky between 2009 to 2012. Megan Bedell, a postdoc at the CCA, spent the first hours after Gaia’s data release combining the Gaia and Kepler data, which will allow anyone to examine potential connections between the two. Using Gaia’s data on stellar motions, she hopes to identify planet-hosting Kepler stars that likely shared a common stellar nursery, then compare their planetary systems. “If stars born together from the same nebula have very different planetary systems, that could tell us new things about planet formation,” she says.

Ruth Angus, meanwhile, a Columbia University postdoc attending the CCA’s event, is studying Gaia data to learn how Kepler’s planetary systems may be changing over time. Assuming that younger stars are closely aligned with the disk of the Milky Way where they recently formed, and that older stars will be in more widely dispersed orbits as a result of gravitational interactions, Angus is attempting to date the planet-hosting stars in Kepler’s field of view, then looking for patterns in their distribution of worlds. “It may be that the overall number of planets per system slowly goes down,” Angus says. “Planets occasionally collide with each other, or even expel each other from their systems. We might then expect more planets on average around young stars than there are around old ones.” Her tentative results, as of the afternoon, suggest this is in fact the case, revealing a preponderance of planets around Kepler stars the Gaia data flags as relatively youthful.


“Gaia is letting us do amazing new things across all of astrophysics,” says CCA director David Spergel. “We now have a thousand times more data at a hundred times the precision on where stars are in and around our galaxy, and this much richer dataset demands new computational methods, both in terms of mining and modeling the data.” Working with CCA postdoc Chiara Mingarelli in the hours following the dataset’s release, Spergel began refining distance estimates to white dwarf stars in orbit around exotic stellar remnants called millisecond pulsars. While somewhat unglamorous, this fundamental work will, among other things, underpin future efforts to use multiple pulsars to detect gravitational waves emitted by merging pairs of supermassive black holes.

Spergel also hopes to use the Gaia data to study dark matter, the mysterious, invisible substance that permeates galaxies, and which is only detectable by its gravitational effects on the stars and gas clouds we can see. “This could be a wonderful tool to track the distribution of dark matter throughout our galaxy, but to do that we need to be able to compute orbits for billions of stars, which is very challenging,” he says. “Achieving that would help us learn whether the dark matter has substantial substructure—in other words, whether it is clumpy.”

Astrophysicists analyzing Gaia data at the CCA event may have already seen hints of such clumps. Adrian Price-Whelan, a postdoc at Princeton, together with Ana Bonaca, a postdoc at Harvard, have used the data to zoom in on a “tidal stream” of stars called GD-1, which lurks in the Milky Way’s halo—a spherical, relatively star-bereft region far beyond the galactic disk. GD-1 and other stellar streams in the halo are thought to be the dismembered remains of globular clusters or dwarf galaxies disrupted by passing through the Milky Way’s gravitational field. In large part because of their paucity of stars, galactic halos are also where theorists believe dark matter’s gravitational influence becomes dominant. The Gaia data confirms and elucidates substructures previously just tenuously glimpsed in GD-1—chiefly a yawning gap in the stream where something seems to have chomped away a massive chunk of stars.

“There are not that many things that can cause a gap like this in a stellar stream,” Bonaca says. One possibility would be a past interaction with a giant molecular cloud, presuming that the stream’s orbit at some point passes near or through the galactic disk. The more alluring option, Price-Whelan and Bonaca say, is that the missing chunk of stars was torn away by a passing clump of dark matter in the galactic halo. “The gap looks large enough to maybe rule out the molecular-cloud hypothesis, although we don’t have exact numbers for that yet,” Price-Whelan says. “If modeling suggests the gap is due to something significantly bigger than a million solar masses—well, most people don’t think giant molecular clouds get that big. It would have to be caused by something else.” The pair are requesting rapid follow-up observations on GD-1, using some of the available time on the MDM Observatory coordinated by the CCA.


Gaia’s data could also enlighten researchers about an even deeper cosmic mystery: dark energy, an unknown force that seems to be powering the universe’s accelerating expansion. Scientists discovered dark energy in the late 1990s, when measurements of exploding stars in faraway galaxies revealed the galaxies to be significantly farther away than previously suspected—a sign of the universe’s dark-energy–driven ballooning growth. Later measurements of the cosmic microwave background—the afterglow of the big bang—also hinted at dark energy’s effects in the early universe.

But the measured magnitude of those primordial effects was some 9 percent lower than the estimates derived from studies of galaxies in the modern cosmos. In essence, the universe appears to be expanding faster than it should be, even when you account for dark energy. That difference may seem insignificant, but is three times larger than the uncertainties associated with each set of measurements, and so cosmologists take it very seriously. Resolving the tension between these two conflicting measurements could reveal the true nature of dark energy, new phenomena beyond the vaunted Standard Model of physics, and even the fate of the universe itself. If, for instance, the tension is a result of dark energy’s effects growing over time, the universe would likely end in a “big rip” in which the fabric of spacetime itself is eventually torn apart by accelerating expansion.

Adam Riess, a cosmologist at Johns Hopkins University who shared the 2011 Nobel Prize in physics for his co-discovery of dark energy, has been trying to resolve the tension for years, using the Hubble Space Telescope to scrutinize a handful of variable stars in far-distant galaxies to better calibrate relatively local (rather than primordial) measurements of dark energy. Now, thanks to Gaia, he has precision measurements for hundreds of suitable variable stars rather than only a handful. “It was always my expectation that Gaia would weigh heavily on the tension between local results and those from the cosmic microwave background,” Riess says.

Working from his office in Maryland rather than the CCA’s conference rooms, he has only just begun to analyze the Gaia dataset, so the answer remains elusive—but Riess believes certainty could come soon. “There are not many situations where I go, ‘today I don’t know, but tomorrow I will,’ about a mystery this big,” he says. “But that’s what I’m hoping for with Gaia. This dataset is like Christmas for those of us working on this cosmological question: a great present, wrapped up and sitting under the tree. Having begun unwrapping it, I look forward to seeing what it has to say.”

And these are only the “known unknowns” astrophysicists hope to resolve with Gaia. There are also, undoubtedly, “unknown unknowns” that will unfold from the observations from this extraordinary satellite. It is a rule of thumb that whenever astronomers get their hands on a powerful new telescope or exponentially larger chunk of data than they had before, discoveries emerge that nobody could have anticipated. Thanks to Gaia, there is every chance that this is about to happen again.


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