Find Your Own Exoplanet

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3 Target Nearby, Young Stars Pluto is about
40 AU (1 AU 1 Astronomical Unit the earth-sun
distance) from the sun, so it is reasonable to
look for planets at tens to hundreds of AU.
However modern telescopes, even when equipped
with AO, have trouble finding extremely faint
objects within 1" (1" 1 arcsecond 1/3600
degree) of a target star. At a distance of 50 pc
(1 parsec 3.26 light years), 1" of angular
separation is equivalent to a linear distance of
50 AU. Thus, 50 pc is a good limiting distance
for target stars. It's also important to choose
stars that are young. The reason is that
planetary or brown dwarf companions grow dim over
time. They are not massive enough to fuse
hydrogen and shine in the manner of stars.
Instead substellar objects mainly radiate the
energy left over from their initial formation.
Once that is gone, planets and brown dwarfs
become too cool and dim for us to detect. That's
why youth is important. An age range of 10 to 100
Myr (1 Myr 106 yr) is a good place to start.
Nearby stars with ages of 1 to 10 Myr would be
even better, but these are extremely rare.
Unfortunately, its often difficult to tell
whether or not a star is young. One useful
indicator is the presence of lithium. This rare
element is quickly destroyed in a secondary
reaction to hydrogen fusion. If a star's spectrum
has a lithium signature, it's a good bet that the
star is young. X-ray flux, and hydrogen line
emission are two other indicators of youth. Also,
a growing pool of evidence shows that a star's
space motions, or 3-D trajectory with respect to
the galactic center, is the same for many young
stars. However, none of these attributes alone
can precisely measure the age of a star and few
stars have been checked for more than one of
these indicators. 4 Verify Possible
Companions Targeting young, nearby stars with a
large, AO-equipped telescope that is optimized to
work in the near infrared is likely to yield many
interesting point sources, as in the case of the
faint object TWA 6B, pictured in Figure 3. But
without independent information about the
distance of the faint object, it is difficult to
tell whether it is really associated with the
target star. It's important to know how to
exclude background objects such as stars or
galaxies. A quick way to determine distance is
to obtain a color observe the faint object at
two different wavelengths, calculate the
brightness at each, and then subtract the two
values. Planets and brown dwarfs have surface
temperatures cooler than 1800 K and will appear
redder (brighter at long wavelengths) than stars.
However, the definitive way to verify
companionship is by seeing whether the star and
the faint object move together in the sky. If the
two are in orbit, the distance between them will
remain roughly the same, but if the two objects
are not gravitationally bound then their
separation will increase over time. Figure 4
shows such a diagram for TWA 6 and its suspected
companion. The measurements fall between the
range expected for a companion or a bound object.
As of yet, no one knows whether TWA 6B is a
planet 1-2 times the mass of Jupiter or an
ordinary background star. Further astrometric
measurements will be the deciding factor.
Denise Kaisler, UCLA
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Over fifty extrasolar planets are now known to
orbit nearby sun-like stars, however, none of
these planets has ever been directly detected.
But now, by using large telescopes with adaptive
optics technology to target young stars in the
solar neighbourhood, it is possible to obtain
actual images of Jupiter-mass exoplanets. Want to
find out how? Then read on! 1 Discover the
Power of AO Adaptive optics (AO) is currently
the most promising way to directly detect
extrasolar planets. This technology not only
increases the sensitivity of ground-based
telescopes, but it produces images of
unprecedented clarity (in astro-speak increased
resolution). AO is particularly effective in
searching for substellar companions to nearby
stars because of the way that it concentrates the
light of point sources. Figure 1 shows a radial
profile of a star and a faint companion. Without
AO, the fainter object is lost in the halo of the
primary, but with an AO system up and running,
the fainter object pops into view. An AO-equipped
telescope can reveal companions up to one million
times fainter than the target star.
2 Observe In the
Near-Infrared Planets give off light of many
wavelengths. They are visible in the optical (0.4
µm to 0.7 µm 1 µm 1 micron 10-6 m),
near-infrared (0.7 µm to 5 µm), and thermal
infrared ( 5 µm) regions of the spectrum, and
also at longer wavelengths. As it turns out,
wavelengths of 1-2 microns are best for detecting
planets with AO. Not only do adaptive optics
systems perform well in this régime, but the
earth's atmosphere is almost completely
transparent at these wavelengths -- a condition
which does not apply to all types of radiation.
Giant planets and their more massive cousins
brown dwarfs not only reflect the infrared (IR)
radiation of their host stars -- they also
produce their own IR radiation. Figure 2 shows
the intensity as a function of wavelength for a
cool star and its cooler, fainter companion. At
optical wavelengths a planet would be lost in the
glare of its host. In the mid- to far infrared,
the sky becomes opaque and large telescopes
become limited by the wavelike nature of light,
eliminating the need for AO. This makes the 1-2
micron range ideal for planet detection.
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Find out more at the Find Your Own Exoplanet
website www.astro.ucla.edu/kaisler/fyoe/
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