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This illustration depicts a newly discovered brown
dwarf, an object that weighs in somewhere between our solar system's
most massive planet (Jupiter) and the least-massive known star. Credits: NASA/JPL-Caltech.Full image and caption
Two space-based telescopes teamed up with ground-based observatories to observe a microlensing event caused by a brown dwarf. Credits: NASA/JPL-Caltech.Full image and caption
In a first-of-its-kind collaboration, NASA's Spitzer and Swift space
telescopes joined forces to observe a microlensing event, when a distant
star brightens due to the gravitational field of at least one
foreground cosmic object. This technique is useful for finding low-mass
bodies orbiting stars, such as planets. In this case, the observations
revealed a brown dwarf.
Brown dwarfs are thought to be the missing link between planets and
stars, with masses up to 80 times that of Jupiter. But their centers are
not hot or dense enough to generate energy through nuclear fusion the
way stars do. Curiously, scientists have found that, for stars roughly
the mass of our sun, less than 1 percent have a brown dwarf orbiting
within 3 AU (1 AU is the distance between Earth and the sun). This
phenomenon is called the "brown dwarf desert."
The newly discovered brown dwarf, which orbits a host star, may
inhabit this desert. Spitzer and Swift observed the microlensing event
after being tipped off by ground-based microlensing surveys, including
the Optical Gravitational Lensing Experiment (OGLE). The discovery of
this brown dwarf, with the unwieldy name OGLE-2015-BLG-1319, marks the
first time two space telescopes have collaborated to observe a
"We want to understand how brown dwarfs form around stars, and why
there is a gap in where they are found relative to their host stars,"
said Yossi Shvartzvald, a NASA postdoctoral fellow based at NASA's Jet
Propulsion Laboratory, Pasadena, California, and lead author of a study published in the Astrophysical Journal. "It's possible that the 'desert' is not as dry as we think."
What is microlensing?
In a microlensing event, a background source star serves as a
flashlight for the observer. When a massive object passes in front of
the background star along the line of sight, the background star
brightens because the foreground object deflects and focuses the light
from the background source star. Depending on the mass and alignment of
the intervening object, the background star can briefly appear thousands
of times brighter.
One way to understand better the properties of the lensing system is
to observe the microlensing event from more than one vantage point. By
having multiple telescopes record the brightening of the background
star, scientists can take advantage of "parallax," the apparent
difference in position of an object as seen from two points in space.
When you hold your thumb in front of your nose and close your left eye,
then open it and close your right eye, your thumb seems to move in space
-- but it stays put with two eyes open. In the context of microlensing,
observing the same event from two or more widely separated locations
will result in different magnification patterns.
"Anytime you have multiple observing locations, such as Earth and
one, or in this case, two space telescopes, it's like having multiple
eyes to see how far away something is," Shvartzvald said. "From models
for how microlensing works, we can then use this to calculate the
relationship between the mass of the object and its distance."
The new study
Spitzer observed the binary system containing the brown dwarf in July
2015, during the last two weeks of the space telescope's microlensing
campaign for that year.
While Spitzer is over 1 AU away from Earth in an Earth-trailing orbit
around the sun, Swift is in a low Earth orbit encircling our planet.
Swift also saw the binary system in late June 2015 through microlensing,
representing the first time this telescope had observed a microlensing
event. But Swift is not far enough away from ground-based telescopes to
get a significantly different view of this particular event, so no
parallax was measured between the two. This gives scientists insights
into the limits of the telescope's capabilities for certain types of
objects and distances.
"Our simulations suggest that Swift could measure this parallax for
nearby, less massive objects, including 'free-floating planets,' which
do not orbit stars," Shvartzvald said.
By combining data from these space-based and ground-based telescopes,
researchers determined that the newly discovered brown dwarf is between
30 and 65 Jupiter masses. They also found that the brown dwarf orbits a
K dwarf, a type of star that tends to have about half the mass of the
Researchers found two possible distances between the brown dwarf
and its host star, based on available data: 0.25 AU and 45 AU. The 0.25
AU distance would put this system in the brown dwarf desert.
"In the future, we hope to have more observations of microlensing
events from multiple viewing perspectives, allowing us to probe further
the characteristics of brown dwarfs and planetary systems," said
Geoffrey Bryden, JPL scientist and co-author of the study.
JPL manages the Spitzer Space Telescope mission for NASA's Science
Mission Directorate, Washington. Science operations are conducted at the
Spitzer Science Center at Caltech in Pasadena, California. 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. NASA's
Swift satellite was launched in November 2004 and is managed by NASA's
Goddard Space Flight Center in Greenbelt, Maryland.