A dusty planetary system (left) is compared to another system with
little dust in this artist's conception. Dust can make it difficult for
telescopes to image planets because light from the dust can outshine
that of the planets. Dust reflects visible light and shines with its own
infrared, or thermal, glow. As the illustration shows, planets appear
more readily in the planetary system shown at right with less dust.
Research with the NASA-funded Keck Interferometer, a former NASA key
science project that combined the power of the twin telescopes of the W.
M. Keck Observatory atop Mauna Kea, Hawaii, shows that mature, sun-like
stars appear to be, on average, not all that dusty. This is good news
for future space missions wanting to take detailed pictures of planets
like Earth and seek out possible signs of life. Image credit: NASA/JPL-Caltech. Full image
MAUNA
KEA, Hawaii — A
new study from the Keck Interferometer, a former NASA project that combined the
power of the twin W. M. Keck Observatory telescopes atop Mauna Kea, Hawaii, has
brought exciting news to planet hunters. After surveying nearly 50 stars from
2008 to 2011, scientists have been able to determine with remarkable precision
how much dust is around distant stars – a big step closer into finding planets
than might harbor life. The discovery is being published in the Astrophysical Journal
online, on December 8th.
“This
was really a mathematical tour de force,” said Peter Wizinowich, Interferometer
Project Manager for Keck Observatory. “This team did something that we seldom
see in terms of using all the available statistical techniques to evaluate the
combined data set. They were able to dramatically reduce all the error bars, by
a factor of 10, to really understand the amount of dust around these systems.”
The Keck
Interferometer was built to seek out this dust, and to ultimately help select
targets for future NASA Earth-like planet-finding missions.
Like
planets, dust near a star is hard to see. Interferometry is a
high-resolution imaging technique that can be used to block out a star's light,
making the region easier to observe. Light waves from the precise
location of a star, collected separately by the twin 10-meter Keck Observatory
telescopes, are combined and canceled out in a process called nulling.
“If you
don't turn off the star, you are blinded and can't see dust or planets,” said
co-author Rafael Millan-Gabet of NASA's exoplanet Science Institute at the
California Institute of Technology in Pasadena, California, who led the Keck
Interferometer's science operations system.
“Dust is
a double-edged sword when it comes to imaging distant planets,” explained
Bertrand Mennesson, lead author of the study who works at NASA's Jet Propulsion
Laboratory, Pasadena, California. “The presence of dust is a signpost for the
planet formation process, but too much dust can block our view.” Mennesson has
been involved in the Keck Interferometer project since its inception more than
10 years ago, both as a scientist and as the optics lead for one of its
instruments.
“Using
the two Keck telescopes in concert and interfering their light beams, it is
possible to distinguish astronomical objects much closer to each other than
when using a single Keck telescope,” Mennesson said. “However, there is an
additional difficulty when searching for warm dust in the immediate stellar
environment: it generally contributes very little emission compared to the
star, and that is when nulling interferometry comes into play.”
In
addition to requiring high performance from a large number of hardware and
software subsystems, the nuller mode requires them to work smoothly together as
a single, integrated system, according to Mark Colavita, the Keck
Interferometer System Architect. “The nulling mode of the interferometer uses
starlight across a wide range of wavelengths, including visible light for the
adaptive optics to correct the telescope wave-fronts, near-infrared light to
stabilize the path-lengths, and mid-infrared light for the nulling science
measurements.”
Planet Hunting
Ground-
and space-based telescopes have already captured images of exoplanets, or planets
orbiting stars beyond our sun. These early images, which show giant planets in
cool orbits far from the glow of their stars, represent a huge technological
leap. The glare from stars can overwhelm the light of planets, like a firefly
buzzing across the sun. So, researchers have developed complex instruments to
block the starlight, allowing information about a planet's shine to be
obtained.
The next
challenge is to image smaller planets in the “habitable” zone around stars
where possible life-bearing Earth-like planets outside the solar system could
reside. Such a lofty goal may take decades, but researchers are already on the
path to get there, developing new instruments and analyzing the dust
kicked up around stars to better understand how to snap crisp planetary
portraits. Scientists want to find out: Which stars have the most dust? And how
dusty are the habitable zones of sun-like stars?
In the
latest study, nearly 50 mature, sun-like stars were analyzed with high
precision to search for warm, room-temperature dust in their habitable zones.
Roughly half of the stars selected for the study had previously shown no signs
of cool dust circling in their outer reaches. This outer dust is easier to see
than the inner, warm dust due to its greater distance from the star. Of this
first group of stars, none were found to host the warm dust, making them good
targets for planet imaging, and a good indication that other relatively
dust-free stars are out there.
The
other stars in the study were already known to have significant amounts
of
distant cold dust orbiting them. In this group, many of the stars were
found to
also have the room-temperature dust. This is the first time a link
between the
cold and warm dust has been established. In other words, if a star is
observed
to have a cold belt of dust, astronomers can make an educated guess that
its
warm habitable zone is also riddled with dust, making it a poor target
for imaging smaller planets in the 'habitable zone' around stars,
or exo-Earths.
“We want
to avoid planets that are buried in dust,” said Mennesson.
Like a
busy construction site, the process of building planets is messy. It's common
for young, developing star systems to be covered in dust. Proto-planets
collide, scattering dust. But eventually, the chaos settles and the dust clears
– except in some older stars. Why are these mature stars still laden with warm
dust in their habitable zones?
The
newfound link between cold and warm dust belts helps answer this question.
“The
outer belt is somehow feeding material into the inner warm belt,” said Geoff
Bryden of JPL, a co-author of the study. “This transport of material could be
accomplished as dust
smoothly flows inward, or there could be larger cometary bodies thrown directly
into the inner system.”
The Keck
Interferometer began construction in 1997, and finished its mission in 2012. It
was developed by JPL, the Keck Observatory and the NASA Exoplanet Science
Institute at Caltech. It was funded by NASA as a part of the Exoplanet
Exploration Program with telescope and instrument operations managed by the W.
M. Keck Observatory.
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 Mauna Kea 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.
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.
SCIENCE CONTACT:
Bertrand Mennesson, PhD
Jet Propulsion Laboratory
(818)-354-0494
bertrand.mennesson@jpl.nasa.gov
MEDIA CONTACT
Steve Jefferson
Communications Officer
W. M. Keck Observatory
808.881.3827
sjefferson@keck.hawaii.edu
Source: W.M. Keck Observatory
