900 Encyclopedia of the Solar System
FIGURE 11 The first observed planetary microlensing event:
two photometric data sets (MOA survey in blue and OGLE
survey in red) were combined to produce this lightcurve. The
best fit binary lens model is shown as solid black line. The
planetary companion causes the strong double peaked and 1
week long deviation from the broader microlensing lightcurve
due to its host star.
3.3.4 THE FIRST DIRECT IMAGING CANDIDATES
Also imaging searches for extrasolar planets yielded the
first candidates for successful detections. In two cases, faint
companions were found in deep infrared images taken with
the adaptive optics system at the 8 m VLT of the Euro-
pean Southern Observatory in Chile. Fainter background
sources, mimicking companions, can be ruled out by the
fact that these objects are co-moving with the central star
and are thus gravitationally bound to it.
One candidate was found near a young “failed” star, a
brown dwarf, which itself is not massive enough to start ther-
monuclear reactions in its core. The central object (called
2MASSWJ 1207334-393254) has only∼25 times the mass
of Jupiter and is located at a distance of approximately 230
lightyears. Its location also gives away its age: The brown
dwarf lies within a young star-forming region, the so-called
TW Hydrae association. This region contains young stars
that are estimated to be only 8 million years old. The fainter
(and thus presumably much less massive) companion was
detected 0.8 arcsec away from the brown dwarf (Fig. 12).
At the distance to the brown dwarf, this transforms to a
projected separation of 55 AU. First low-resolution spectra
of the companion were also obtained. But is it a planet?
This is the tricky part to decide. The observed brightness,
colors, and spectral information can be compared to theo-
retical models for young planets. Such a comparison yields
a mass of∼5 Jupiter masses for the fainter companion, a
FIGURE 12 An image of an extrasolar planet? The companion
(red) to the brown dwarf 2MASSWJ1207334–393254 (blue)
might have a mass of only 5 Jupiter masses.mass value placing the object firmly within the range of
planets. However, some caution remains because the theo-
retical models for young planets are not calibrated and their
uncertainties are difficult to assess. Moreover, the large sep-
aration of (at least) 55 AU raises the question how massive
the protoplanetary disk around a 25 Jupiter mass object
must have been to form such a massive planetary compan-
ion so far away from the center.
Another candidate for a directly imaged extrasolar planet
is the companion to the star GQ Lupi. This time the central
object is really a star similar to the Sun, albeit a lot younger.
The age of GQ Lupi is estimated to be between 100,000 and
2 million years. These very young ages for the imaging can-
didates result from an observational selection effect: Imag-
ing searches specifically target young stars because young
planets are much brighter at these early evolutionary stages.
The companion appears 0.7 arcsec to the west of GQ Lupi
(see Fig. 13), which translates into a projected separation
of∼100 AU. A careful comparison of the discovery images
with archived images revealed that the companion is indeed
a co-moving object (they share the same motion in the plane
of the sky, which rules out a background object). Therefore,
it can be assumed that it has formed at the same time as the
star GQ Lupi has. In this case, the comparison of the avail-
able photometric and spectral data with models for young
planets yielded a mass between 1 and 3 Jupiter masses for
the companion. But again, the remaining uncertainties in
the values derived from models are large, and it is difficult