Watery
asteroid discovered in dying star points to habitable exoplanets
Artist's impression of a rocky and water-rich
asteroid being torn apart by the strong gravity of thewhite dwarf star GD
61. Similar objects in the solar system likely delivered the bulk of water on
Earth and represent the building blocks of the terrestrial planets. Credit: ©
Mark A. Garlick, space-art.co.uk, University of Warwick and University of
Cambridge
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Astronomers have found the shattered remains of an asteroid
that contained huge amounts of water orbiting an exhausted star,
or white dwarf. This suggests that the star GD 61 and itsplanetary system –
located about 150 light years away and at the end of its life – had the
potential to contain Earth-like exoplanets, they say.
This is the first time that both water and a rocky surface - two "key ingredients" for habitable planets - have been found together beyond our solar system.
This is the first time that both water and a rocky surface - two "key ingredients" for habitable planets - have been found together beyond our solar system.
Earth is essentially a 'dry' planet, with only 0.02% 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 latest evidence points to it containing a similar type of
water-rich asteroid that would have first brought water to Earth.
The asteroid analysed is composed of 26% water mass, very
similar to Ceres, the largest asteroid in the main belt of our solar
system. Both are vastly more water-rich compared with Earth.
Astronomers at the Universities of Cambridge and
Warwick say this is the first "reliable evidence" for water-rich,
rocky planetary material in any extrasolar planetary system.
They describe it as a "look into our future" as,
six billion years from now, alien astronomers studying the
rocky remains around our burned out sun might reach the same
conclusion - that terrestrial planets once circled our parent
star.
The new research findings used NASA's Hubble
Space Telescope and are reported today in the journal Science.
All rocky planets form from the accumulation of asteroids,
growing until full size, so asteroids are essentially the 'building blocks' of
planets.
"The finding of water in a large asteroid means the
building blocks of habitable planets existed – and maybe still exist – in the
GD 61 system, and likely also around substantial number of similarparent stars,"
said lead author Jay Farihi, from Cambridge's Institute of Astronomy.
"These water-rich building blocks, and the terrestrial
planets they build, 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," Farihi said.
"Our results demonstrate that there was definitely
potential for habitable planets in this exoplanetary system."
The researchers say that the water detected most likely came
from a minor planet, at least 90 km in diameter but probably much larger, that
once orbited the GD 61 star before it became a white dwarf around 200 million
years ago.
Previous and current astronomical observations have measured
the size and density of exoplanets, but not their composition.
This is because conventional work was done on planets
orbiting living stars. But the only way to see what a distant planet
is made of is to take it apart, say the researchers, and nature does this for
us in a dying white dwarf system through its extreme gravitational pull –
sucking in and shredding the surrounding material.
This debris, which "pollutes" the atmosphere of
the white dwarf, can then be chemically analysed using powerful spectrograph
techniques that "distill the entire asteroid, core and all", they
say.
The team detected a range of "elemental abundance"
in the white dwarf's contaminated atmosphere – such as magnesium, silicon and
iron, which, together with oxygen are the main components of rocks.
By calculating 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 – but they found "significantly" more
oxygen than if
there were only rocks.
"This oxygen excess can be carried by either water or
carbon, and in this star there is virtually no carbon – indicating there must
have been substantial water," said co-author Boris Gänsicke, from the University
of Warwick.
"This also rules out comets, which are rich in both
water and carbon compounds, so we knew we were looking at a rocky asteroid with
substantial water content – perhaps in the form of subsurface ice –
like the asteroids we know in our solar system such as Ceres," Gänsicke
said.
Ultraviolet observations are the only way to obtain such
precise measurement of oxygen levels in the white dwarf's debris – and that can
only be carried out above the Earth's atmosphere.
The team used the Cosmic Origins Spectrograph onboard Hubble
to get the data required, with chemical analysis computed by team member Detlev
Koester from the University of Kiel.
The "planetary bodies" such as these asteroids
that fall into and pollute this dying star – which, in its heyday, was three
times 'heavier' than our sun – also reveal that giant exoplanets probably still
exist in this remote and withering system.
"In order for the asteroids to pass sufficiently close
to the white dwarf to be shredded, then eaten, they must perturbed from the
asteroid belt – essentially pushed – by a massive object like a giant
planet," added Farihi.
"These asteroids tell us that the GD 61 system had – or
still has – rocky, terrestrial planets, and the way they pollute the white
dwarf tells us that giant planets probably still exist there.
"This supports the idea that the star originally had a
full complement of terrestrial planets, and probably gas giant planets,
orbiting it – a complex system similar to our own."
Source: phys.org
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