Sky & Telescope - February 2023 - 39

ESA / GAIA / DPAC / CC BY-SA 3.0 IGO
ASTEROIDS GALORE
The position of each asteroid
Gaia has detected within
Jupiter's orbit, plotted for the
date of Data Release 3 (June
13, 2022). Each segment represents
the asteroid's motion
over 10 days. Blue represents
the inner solar system, green
the main asteroid belt, and
orange the Trojans that accompany
Jupiter. Earth, Jupiter,
and the Sun are shown for orientation
and are not to scale.
and diameter of the parent star. Thanks to Gaia, it has been
possible to derive much more precise data on the physical
properties of many known exoplanets.
Much closer to home, Gaia is revolutionizing asteroid
research. DR3 contains data on 158,152 asteroids, Kuiper Belt
objects, and planetary satellites, including spectra of 60,518
of these small solar system bodies. " This is simply the largest
spectroscopic survey ever published, " says Paolo Tanga (Côte
d'Azur Observatory). " I am sure that many discoveries will
come from this: on the composition of ancient planetesimals
fragmented by collisions into asteroid families, on the formation
of the asteroid belt, and maybe on asteroid satellites that
can be found through astrometry. " According to Federica
Spoto (Center for Astrophysics, Harvard & Smithsonian),
Gaia's " ultra-accurate asteroid positions " are about 100 times
more precise than earlier ground- or space-based measurements.
" It's really a new way to see the solar system, " she said
at the presentation of DR3.
Galactic Archaeology
Some of the most exciting Gaia results to date are related to
the evolution of our Milky Way Galaxy. Over time, the Milky
Way has grown to its present bulk by " consuming " smaller
satellite galaxies and globular clusters. But even after hundreds
of millions or even billions of years, stars that share
a common origin have similar orbital energies and angular
momenta, both of which are so-called conserved quantities,
explains Amina Helmi (University of Groningen, the Netherlands).
So astronomers can reconstruct the Milky Way's
complicated formation history by precisely mapping the
motions and compositions of its constituent stars (S&T: Mar.
2020, p. 34).
In April 2018, within hours of Gaia's second data release,
Helmi and her collaborators discovered the telltale dynamical
fingerprint of a massive galaxy merger that occurred some 10
billion years ago. They nicknamed this galaxy Gaia-Enceladus,
after the mythological giant (and son of the Earth goddess
Gaia) who was buried under Sicily's Mount Etna, driving its
volcanic activity. Likewise, the merger with Gaia-Enceladus
produced an eruption of star formation in the Milky Way.
The stars from Gaia-Enceladus (sometimes now called
Gaia-Sausage-Enceladus, or GSE) are found throughout our
galaxy and in its halo. But more recent mergers have left
elongated streams of stars with similar dynamical properties
and compositions. More than 20 years ago, Helmi had already
found such stellar streams in data from the earlier Hipparcos
mission. While these so-called Helmi streams were recognized
on the basis of just 13 individual stars, Gaia's much greater
sensitivity has revealed similar streams of many hundreds of
stars, some of which extend over tens of degrees of sky.
" We are now able to find the remains of much smaller and
more recent mergers, " says Helmi. " Dwarf galaxy streams
have typical widths of hundreds of light-years, but we've also
discovered streams that are less than 10 light-years wide -
the tidally disrupted remains of globular clusters. " Eventually,
Helmi hopes to use these streams to fully reconstruct the
Milky Way's formation history.
Studying stellar streams - which usually extend well into
the Milky Way's halo - may also shed light on the mystery of
dark matter. According to computer simulations, the galactic
sk yand tele scope .o r g * FEBRUARY 2023 39
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