Artist's
impression of the surface of the massive, planet-like body being
devoured by the white dwarf SDSSJ1043+0855. The Keck Observatory and
Hubble Space Telescope data (shown in inset) show calcium and carbon,
the presence of which can be explained with a model suggesting the
surface of the planet may have been encrusted in limestone
(calcium-carbonate). This material was removed from the surface of the
massive rocky body, probably through large-scale collisions,
subsequently shredded into a disk of material, and accreted by the white
dwarf star (ringed object seen in the planet's sky). Credit: A. Hara/C. Melis/W. M. Keck Observatory
MAUNAKEA, HI – A group of researchers using the W. M. Keck Observatory have discovered a planet-like body that may have been encrusted in limestone and is having its surface layers devoured by its deceased host star. In addition to extending a relatively new method of determining the chemical composition of planets to examine their internal structure, the team found that the rocky material being accreted by the star could be comprised of minerals that are typically associated with marine life processes here on Earth.
The team –
comprised of Carl Melis of University of California, San Diego and
Patrick Dufour of the Universitie de Montreal – is announcing their
findings at the 228th meeting of the American Astronomical Society this week.
Building
on past observations of the white dwarf called SDSSJ1043+0855 (the dead
core of a star that originally was a few times the mass of the Sun),
which has been known to be gobbling up rocky material in its orbit for
almost a decade, the team used Keck Observatory’s HIRES instrument
fitted to the 10-meter Keck I telescope as well as data from the Hubble
Space Telescope to measure and characterize the material being accreted
by the star.
What they found is that the white dwarf appears to be
accreting the outer-most layers of a differentiated, rocky extrasolar
body (i.e., the surface of massive, planet-like object) from its extant
planetary system.
“Spectroscopic observations of the white dwarf
allowed us to measure the abundances of the rocky material as it is
being accreted and filtered through the star’s atmosphere in real time,”
Melis said. “We can see the material that used to make up this planet
being accreted and replenished on a daily timescale. What we see is what
the rock was made of.”
This may be the single best tool
astronomers have to determine the chemical composition of planets,
according to Luca Rizzi, Support Astronomer at Keck Observatory.
“We've
known for some time that examining the accreted remains of rocky
planets in the atmosphere of their host white dwarf star can give bulk
chemical composition information, and now it looks like we can even hone
in on specific layers of an accreted body in some fortuitous cases,”
Melis said.
Determining the chemical composition or structure of
planets outside of the Solar system to date has been elusive at best.
“It’s a huge issue in exo-planetology right now,” Melis said. “The major
exoplanet identifying methods can't tell you what a planet is made of
or what it's structure is.”
While the finding will provide a new
angle for scientists to study the chemical composition and structure of
rocky planets, the possibility that life may have contributed to the
inferred mineralogy certainly intrigued the team.
The researchers’
finding shows that SDSSJ1043+0855 is accreting the surface of a body
that has large enhancements of carbon. This feature — combined with mild
enhancements of calcium and oxygen — points to the possibility of the
material coming in the form of calcium-carbonate, a mineral that is
often associated with shelled marine organisms here on Earth.
Calcium-carbonate is attractive as a mineral constituent of this
planet-like body as incorporating and entraining carbon in rocky objects
(especially their surfaces) is difficult. The terrestrial planets in
our Solar system are said to live in a “carbon desert” since they are so
heavily depleted in this element — the planetary surface being accreted
by this white dwarf star could have as much as several hundred times
more carbon than the surface of the Earth.
“This method allows us
to truly get a glimpse of what aliens might be standing on,” Melis said.
“In this particular case, the presence of such high levels of carbon is
unique and really needs to be explained. Our choice of
calcium-carbonate as a potential carrier of the carbon provides a
natural way for it to be locked up in the planet and eventually
delivered to the white dwarf star, is entirely consistent with the
observations in hand, and of course is suggestive. That’s really the
hidden subtext. When people think about finding extra-terrestrial life,
they think about Hollywood dramatizations. But the first evidence of
life outside of our Solar system will probably come in a much subtler
form. More likely than not, it’s going to come as a nuanced signature
that may not be immediately recognizable.”
Non-biological
processes can produce calcium-carbonate too, so its presence isn't
necessarily a smoking gun, even if it is confirmed. “There’s a lot of
hoops to jump through before we can settle on the conclusion that life
was involved in what we are observing,” Dufour said.
Specifically,
the inferred presence of calcium-carbonate came from examining the
atomic leftovers of the planet accretion event in the atmosphere of the
white dwarf star – after the presumed dust from the planet’s demolished
surface was consumed by the white dwarf. The next step will be to look
at the dust in a mineral state before it falls into the star, to both
confirm its composition and to measure its concentration.
“Future
observations with the James Webb Space Telescope can confirm
calcium-carbonate if it is present. If we are able to get to that point,
then you have to ask: Is there enough there for it to be produced with
natural processes?” Melis said.
While the presence of the
calcium-carbonate is still in question, the paper shows strong evidence
that the accreted material is almost certainly coming from the outer
layers of a planet-like object and that white dwarf stars hold promise
in informing on the structure of planets outside of the Solar system.
The
W. M. Keck Observatory operates the largest, most scientifically
productive telescopes on Earth. The two, 10-meter optical/infrared
telescopes near the summit of Maunakea on the Island of Hawaii feature a
suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field
spectrographs and world-leading laser guide star adaptive optics
systems.
HIRES (the High-Resolution Echelle Spectrometer)
produces spectra of single objects at very high spectral resolution, yet
covering a wide wavelength range. It does this by separating the light
into many "stripes" of spectra stacked across a mosaic of three large
CCD detectors. HIRES is famous for finding planets orbiting other stars.
Astronomers also use HIRES to study distant galaxies and quasars,
finding clues to the Big Bang.
Keck Observatory is a private
501(c) 3 non-profit organization and a scientific partnership of the
California Institute of Technology, the University of California and
NASA.
By Steve Jefferson
Media Contact
Steve Jefferson
W. M. Keck Observatory
(808) 881-3827
sjefferson@keck.hawaii.edu
Science Contact
Carl Melis
University of California, San Diego
(858) 534-6627
cmelis@ucsd.edu
Source: W.M. Keck Observatory
