May 9, 2013
Hubble Space Telescope has found signs of
Earth-like planets in an unlikely place: the
atmospheres of a pair of burnt-out stars in
a nearby star cluster.
The white dwarf
stars are being polluted by debris from
asteroid-like objects falling onto them.
suggests that rocky planet assembly is
common in clusters, say researchers.
of debris around a white dwarf star.
(Credit: NASA, ESA,
STScI, and G. Bacon - STScI)
The stars, known as white dwarfs -
small, dim remnants of stars once like the Sun - reside 150
light-years away in
the Hyades star cluster, in the
constellation of Taurus (The Bull).
The cluster is relatively young, at only 625 million years old.
Astronomers believe that all stars formed in clusters.
However, searches for planets in these
clusters have not been fruitful - of the roughly 800
exoplanets known, only four are
known to orbit stars in clusters. This scarcity may be due to the
nature of the cluster stars, which are young and active, producing
stellar flares and other outbursts that make it difficult to study
them in detail.
A new study led by Jay Farihi of the University of Cambridge,
UK, instead observed "retired" cluster stars to hunt for signs of
planet formation. 
Hubble's spectroscopic observations identified silicon in the
atmospheres of two white dwarfs, a major ingredient of the rocky
material that forms Earth and other terrestrial planets in the Solar
System. This silicon may have come from asteroids that were shredded
by the white dwarfs' gravity when they veered too close to the
stars. The rocky debris likely formed a ring around the dead stars,
which then funneled the material inwards.
The debris detected whirling around the white dwarfs suggests that
terrestrial planets formed when these stars were born.
After the stars collapsed to form white
dwarfs, surviving gas giant planets may have gravitationally nudged
members of any leftover asteroid belts into star-grazing orbits.
"We have identified chemical
evidence for the building blocks of rocky planets," says Farihi.
"When these stars were born, they
built planets, and there's a good chance that they currently
retain some of them. The signs of rocky debris we are seeing are
evidence of this - it is at least as rocky as the most primitive
terrestrial bodies in our Solar System."
Besides finding silicon in the Hyades
stars' atmospheres, Hubble also detected low levels of carbon.
This is another sign of the rocky nature
of the debris, as astronomers know that carbon levels should be very
low in rocky, Earth-like material.
Finding its faint chemical signature
required Hubble's powerful Cosmic Origins Spectrograph (COS),
as carbon's fingerprints can be detected only in ultraviolet light,
which cannot be observed from ground-based telescopes.
"The one thing the white dwarf
pollution technique gives us that we won't get with any other
planet detection technique is the chemistry of solid planets,"
"Based on the silicon-to-carbon
ratio in our study, for example, we can actually say that this
material is basically Earth-like."
This new study suggests that asteroids
less than 160 kilometers across  were gravitationally
torn apart by the white dwarfs' strong tidal forces, before
eventually falling onto the dead stars. 
The team plans to analyze more white dwarfs using the same technique
to identify not only the rocks' composition, but also their parent
"The beauty of this technique is
that whatever the Universe is doing, we'll be able to measure
it," Farihi said.
"We have been using the Solar System
as a kind of map, but we don't know what the rest of the
Universe does. Hopefully with Hubble and its powerful
ultraviolet-light spectrograph COS, and with the upcoming
ground-based 30- and 40-metre telescopes, we'll be able to tell
more of the story."
 The two "polluted" Hyades white
dwarfs are part of a search of planetary debris around more than
100 white dwarfs, led by Boris Gänsicke of the University of
Warwick, United Kingdom. Using computer models of white dwarf
atmospheres, Detlev Koester from the University of Kiel in
Germany is determining the abundances of various elements that
can be traced to planets in the COS data.
 Seeing evidence of asteroids points to the possibility of
Earth-sized planets in the same system. Asteroids are the
building blocks of major planets. Planet-forming processes are
inefficient, and spawn many times more small bodies than large
bodies - but once rocky embryos the size of asteroids are built,
planets are sure to follow.
 The team estimated the size of the in-falling asteroids by
measuring the amount of dust being gobbled up by the stars -
about 10 million grams per second, equal to the flow rate of a
small river. They then compared that data with measurements of
material falling onto other white dwarfs.
 The Hyades study offers insight into what will happen in the
Solar System when the Sun burns out, five billion years from
Notes for editors
The Hubble Space Telescope is
a project of international cooperation between ESA and NASA.
 The international team of
astronomers in this study consists of J. Farihi (University of
Cambridge, UK; STFC Ernest Rutherford Fellow), B. T. Gänsicke
(University of Warwick, UK), D. Koester (University of Kiel,
 The new study is appearing in the Monthly Notices of the
Royal Astronomical Society.