This infographic explains how NASA's Spitzer Space Telescope can be used
in tandem with a telescope on the ground to measure the distances to
planets discovered using the "microlensing" technique. Credit: NASA/JPL-Caltech/Warsaw University University
This plot shows data obtained from NASA's Spitzer Space Telescope and
the Optical Gravitational Lensing Experiment, or OGLE, telescope
located in Chile, during a "microlensing" event. Microlensing events
occur when one star passes another, and the gravity of the foreground
star causes the distant star's light to magnify and brighten. This
magnification is evident in the plot, as both Spitzer and OGLE register
an increase in the star's brightness.
If the foreground star is
circled by a planet, the planets gravity can alter the magnification
over a shorter period, seen in the plot in the form of spikes and a dip.
The great distance between Spitzer, in space, and OGLE, on the ground,
meant that Spitzer saw this particular microlensing event before OGLE.
The offset in the timing can be used to measure the distance to the
planet.
In this case, the planet, called OGLE-2014-BLG-0124L, was
found to be 13,000 light-years away, near the center of our Milky Way
galaxy.
The finding was the result of fortuitous timing because
Spitzer's overall program to observe microlensing events was only just
starting up in the week before the planet's effects were visible from
Spitzers vantage point.
While Spitzer sees infrared light of 3.6 microns in wavelength, OGLE sees visible light of 0.8 microns. Credit: NASA/JPL-Caltech/Warsaw University University
Most
of the known exoplanets, numbering in the thousands, have been
discovered by NASA's Kepler space telescope, which uses a different
strategy called the transit method. Kepler's cone-shaped field of view
is shown in pink/orange. Ground-based telescopes, which use the transit
and other planet-hunting methods, have discovered many exoplanets close
to home, as shown by the pink/orange circle around the sun.
Credit: NASA/JPL-Caltech
NASA's Spitzer Space Telescope has teamed up with a telescope on the
ground to find a remote gas planet about 13,000 light-years away, making
it one of the most distant planets known.
The discovery
demonstrates that Spitzer -- from its unique perch in space -- can be
used to help solve the puzzle of how planets are distributed throughout
our flat, spiral-shaped Milky Way galaxy. Are they concentrated heavily
in its central hub, or more evenly spread throughout its suburbs?
"We
don't know if planets are more common in our galaxy's central bulge or
the disk of the galaxy, which is why these observations are so
important," said Jennifer Yee of the Harvard-Smithsonian Center for
Astrophysics, Cambridge, Massachusetts, and a NASA Sagan fellow. Yee is
the lead author of one of three new studies that appeared recently in
the Astrophysical Journal describing a collaboration between astronomers
using Spitzer and the Polish Optical Gravitational Lensing Experiment,
or OGLE.
OGLE's Warsaw Telescope at the Las Campanas Observatory
in Chile scans the skies for planets using a method called microlensing.
A microlensing event occurs when one star happens to pass in front of
another, and its gravity acts as a lens to magnify and brighten the more
distant star's light. If that foreground star happens to have a planet
in orbit around it, the planet might cause a blip in the magnification.
Astronomers
are using these blips to find and characterize planets tens of
thousands of light-years away in the central bulge of our galaxy, where
star crossings are more common. Our sun is located in the suburbs of the
galaxy, about two-thirds of the way out from the center. The
microlensing technique as a whole has yielded about 30 planet
discoveries so far, with the farthest residing about 25,000 light-years
away.
"Microlensing experiments are already detecting planets
from the solar neighborhood to almost the center of the Milky Way," said
co-author Andrew Gould of The Ohio State University, Columbus. "And so
they can, in principle, tell us the relative efficiency of planet
formation across this huge expanse of our galaxy."
Microlensing
complements other planet-hunting tools, such as NASA's Kepler mission,
which has found more than 1,000 planets closer to home. But it faces one
key problem: This method can't always precisely narrow down the
distance to the stars and planets being observed. While a passing star
may magnify the light of a more distant star, it rarely can be seen
itself, making the task of measuring how far away it is challenging.
Of
the approximately 30 planets discovered with microlensing so far,
roughly half cannot be pinned down to a precise location. The result is
like a planetary treasure map lacking in X's.
That's where Spitzer
can help out, thanks to its remote Earth-trailing orbit. Spitzer
circles our sun, and is currently about 128 million miles (207 million
kilometers) away from Earth. That's father from Earth than Earth is from
our sun. When Spitzer watches a microlensing event simultaneously with a
telescope on Earth, it sees the star brighten at a different time, due
to the large distance between the two telescopes and their unique
vantage points. This technique is generally referred to as parallax.
"Spitzer
is the first space telescope to make a microlens parallax measurement
for a planet," said Yee. "Traditional parallax techniques that employ
ground-based telescopes are not as effective at such great distances."
Using
space telescopes to observe microlensing events is tricky. Ground
telescopes send out alerts to the astronomy community when an event
starts, but the activity can quickly fade, lasting on average about 40
days. The Spitzer team has scrambled to start microlensing campaigns as
soon as three days after receiving an alert.
In the case of the
newfound planet, the duration of the microlensing event happened to be
unusually long, about 150 days. Both Spitzer and OGLE's telescopes
detected the telltale planetary blip in the magnification, with Spitzer
seeing it 20 days earlier.
This time delay between viewing of the
event by OGLE and Spitzer was used to calculate the distance to the star
and its planet. Knowing the distance allowed the scientists also to
determine the mass of the planet, which is about half that of Jupiter.
Spitzer
has eyed 22 other microlensing events in collaboration with OGLE and
several other ground-based telescopes. While these observations have not
turned up new planets, the data are essential to learning the
population statistics of stars and planets at the heart of our galaxy.
Spitzer will watch approximately 120 additional microlensing events this
summer.
"We've mainly explored our own solar neighborhood so
far," said Sebastiano Calchi Novati, a Visiting Sagan Fellow at NASA's
Exoplanet Science Institute at the California Institute of Technology,
Pasadena. "Now we can use these single lenses to do statistics on
planets as a whole and learn about their distribution in the galaxy."
NASA's
Jet Propulsion Laboratory, Pasadena, California, manages the Spitzer
Space Telescope mission for NASA's Science Mission Directorate,
Washington. Science operations are conducted at the Spitzer Science
Center at the California Institute of Technology in Pasadena. Spacecraft
operations are based at Lockheed Martin Space Systems Company,
Littleton, Colorado. Data are archived at the Infrared Science Archive
housed at the Infrared Processing and Analysis Center at Caltech.
Caltech manages JPL for NASA.
Source: Spitzer/JPL-Caltech
