Artist's View of Watery Asteroid in White Dwarf Star System GD 61
This is an artist's impression of a rocky and water-rich asteroid
being torn apart by the strong gravity of the white dwarf star GD 61.
Similar objects in our solar system likely delivered the bulk of water
on Earth and represent the building blocks of the terrestrial planets. Artwork Credit: NASA, ESA, M.A. Garlick (space-art.co.uk), University of Warwick, and University of Cambridge. Science Credit: NASA, ESA, J. Farihi (University of Cambridge), B. Gänsicke (University of Warwick), and D. Koester (University of Kiel)
Astronomers using NASA's Hubble Space Telescope have found the
building blocks of solid planets that are capable of having substantial
amounts of water. This rocky debris, currently orbiting a white dwarf
star called GD 61, is considered a relic of a planetary system that
survived the burnout of its parent star. The finding suggests that the
star system — located about 150 light-years away and at the end of its
life — had the potential to contain Earth-like exoplanets, the
researchers say.
"These water-rich building blocks, and the terrestrial planets they
assemble, may in fact be common. A system cannot create things as big as
asteroids and avoid building planets, and GD 61 had the ingredients to
deliver lots of water to their surfaces," according to Jay Farihi of the
University of Cambridge, United Kingdom. Though it's hard to predict
exactly what types of planets there might have been, Farihi emphasized
that, "Our results demonstrate that there was definitely potential for
habitable planets in this exoplanetary system. The system almost
certainly had (and possibly still has) planets, and it had the
ingredients to deliver lots of water to their surfaces."
The new research findings are reported today in the journal Science.
Observations made with Hubble's Cosmic Origins Spectrograph (COS)
allowed the team, led by Farihi, to do a robust chemical analysis of the
debris falling into GD 61. The discovery complements other leading
astronomical observations that measure the size and density of planets,
but not their actual composition, say researchers.
"The only feasible way to see what a distant planet is made of is to
take it apart, and nature does this for us using the strong
gravitational tidal forces of white dwarf stars," said Farihi. "This
technique allows us to look at the chemistry that builds rocky planets,
and is a completely independent method from other types of exoplanet
observations."
The white dwarf GD 61 is a relic of a star that once burned hotter
and brighter than our Sun. The star exhausted its fuel in just 1.5
billion years. (Our Sun will last roughly ten times as long.)
NASA's Far Ultraviolet Space Explorer (FUSE) first found an abundance
of oxygen in the dwarf's atmosphere in 2008. Eventually astronomers
realized that this was the telltale signature of material falling into
the star and polluting its atmosphere. White dwarfs typically have pure
hydrogen or pure helium atmospheres. The "polluted white dwarf" scenario
was bolstered by NASA Spitzer Space Telescope observations in 2011,
which showed that the star has a tightly orbiting disk containing debris
that falls onto the star and contaminates the otherwise pristine
atmosphere.
The only way to obtain a more precise measurement of the amount of
oxygen in the debris around GD 61 requires observations in the
ultraviolet, which can only be carried out above Earth's atmosphere. The
team used COS aboard Hubble to obtain the required data. The COS
observations were then analyzed by Detlev Koester of the University of
Kiel, in Germany, using a computer model of the white dwarf atmosphere
to derive the elemental abundances.
Combing their results with a previous study that used the W. M. Keck
Observatory on the summit of Mauna Kea, Hawaii, the team also detected
magnesium, silicon, and iron, which, together with oxygen, are the main
components of rocks. By counting the number of these elements relative
to oxygen the researchers were able to predict how much oxygen should be
in the atmosphere of the white dwarf. They found significantly more
oxygen than should have been carried by rocky minerals alone. "The
oxygen excess can be carried by either water or carbon mono- or dioxide.
In this star there is virtually no carbon, indicating there must have
been substantial water," said Boris Gnsicke of the University of
Warwick, in Coventry, United Kingdom. He added that the small amount of
carbon seen in the white dwarf rules out comets as the source of water.
Comets are rich in both water and carbon compounds.
In their Hubble survey the team observed nearly 100 white dwarfs.
Analysis is still ongoing, but the team estimates that at least 20
percent of the dwarfs show ongoing accretion of planetary debris, and it
could possibly be as high as 50 percent.
How do the asteroids fall into the stellar remnant? The best model at
present is based on how Jupiter perturbs members of our main asteroid
belt. The Kirkwood Gaps in the asteroid belt represent areas where
asteroids lose energy to Jupiter and sometimes fall into the Sun.
Infrared observations using the Spitzer telescope show that Sun-like
stars that are similar to the parent star of GD 61 have inner debris
belts analogous to our main asteroid belt. And, interestingly, these
systems appear to have a gap just outside their inner belts that may be
caused by one or more planets, say the investigators. "It looks like a
pattern of a planet next to an asteroid belt whose members get thrown
into the star may be a common feature of solar systems," said Farihi.
Earth is essentially a "dry" planet, with only 0.02 percent of its
mass as surface water. So oceans came long after it had formed, most
likely when water-rich asteroids in the solar system crashed into our
planet.
The new discovery shows that the same water "delivery system" could
have occurred in this distant, dying star's solar system — as this
latest evidence points to it containing a similar type of water-rich
asteroid that would have first brought water to Earth.
Six billion years from now an alien astronomer measuring similar
abundances in the atmosphere of our burned-out Sun may reach the same
conclusion that terrestrial planets once circled our parent star. Though
the progenitor star was different from our Sun, nevertheless, "it's a
look into our future," said Gänsicke.
CONTACT
Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu
Jay Farihi
University of Cambridge
Institute of Astronomy
Cambridge CB3 0HA
+44 122 333 0896
jfarihi@ast.cam.ac.uk
