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.
Old worlds and new
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 post-doc 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
said.
Ruth Angus, meanwhile, a Columbia University post-doc 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 said. “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.
Searching for dark matter and gravitational waves
“Gaia is letting us do amazing new things across all of
astrophysics,” said 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 post-doc 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
said. “Achieving that would help us learn whether the dark matter has
substantial substructure—in other words, whether it is clumpy.”
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Astrophysicists analyzing Gaia data at the CCA event may have already
seen hints of such clumps. Adrian Price-Whelan, a post-doc at
Princeton, together with Ana Bonaca, a post-doc 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 said. 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 said. “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.
New physics and the fate of the universe
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 said.
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 said. “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.
This article is reproduced with permission from Scientific American. It was first published on April 25, 2018. Find the original story here.
