898 Encyclopedia of the Solar System
TABLE 3 General Characteristics of the 3 Hot Neptunesmsini Orbital Period
Star (Earth mass) P(days) a(AU) Notesρ1 Cancri 14.2 2.81 0.04 4 planet system
μAra 14.5 9.55 0.09 2 planet system
Gliese 436 21.5 2.64 0.03 M dwarf starmade up mostly of rocky material (possibly surrounded by a
small gaseous envelope) and that their discoveries by the ra-
dial velocity technique represents another step toward the
detection of more Earth-like planets.
3.3 Results of the Other Detection Methods
Although the radial velocity technique is by far the most suc-
cessful method and pulsar timing was the very first method
to detect planetary companions to other stars, they are not
the only methods to discover planets. A few extrasolar plan-
ets were also detected by transit searches, microlensing sur-
veys, and possibly even by direct imaging.
3.3.1 TRANSIT SEARCHES
The Optical Gravitational Lensing Experiment (OGLE) is
a precise photometric survey of millions of stars to search
for gravitational lensing events. OGLE is a project from
astronomers from Warsaw and Princeton and operates a
1.3 m telescope at Las Campanas Observatory in Chile.
The OGLE data can also be used to search for the charac-
teristic flat-bottomed lightcurves of planetary transits. For
more than 50,000 stars in the OGLE fields, the quality of
the photometry is sufficient to perform this task. In total,
more than 100 transit-like events were found in the OGLE
data. However, there are a large number of other astrophys-
ical phenomena that can mimic the photometric signal of
a planetary transit. Thus, in order to determine whether a
transiting object is indeed a planet, it is necessary to ob-
tain spectroscopic follow-up observations to characterize
the host star and subsequently to perform radial velocity
measurements to derive a mass for the companion. The
majority of the OGLE candidates turned out to be false
alarms, mostly binary systems with a very hot (or giant) pri-
mary star and a very cool secondary star, or binary stars
undergoing grazing eclipses.
The first transit candidate which was confirmed as a
planet by radial velocity observations was OGLE-TR-56.
This object was thus the very first planet discovered by the
transit method (HD 209458 b, the first transiting planet, was
detected by radial velocities before the photometric tran-
sit was observed). The companion to OGLE-TR-56 has a
mass of 1.4 Jupiter masses and a radius of 1.2 Jupiter radii.
The planet has an extremely short orbital period of only
1.2 days or 29 hours, and the orbital separation is only 0.023
AU. This is even closer to the host star than the hot Jupiters
found by radial velocity surveys. Four more transiting plan-
ets were revealed by the OGLE data, two of them are “very
hot Jupiters” similar to OGLE-TR-56, while the other two
have orbital periods typical for hot Jupiters.
In order to survey a sufficiently large number of stars,
OGLE monitors areas close to the galactic center. Most of
the stars included in these fields are far more distant and
thus fainter than the stars included in radial velocity surveys.
This makes the spectroscopic follow-up observations more
problematic. Large aperture telescopes like the 10 m Keck
telescope in Hawaii or the8mVeryLarge Telescope (VLT)
in Chile are necessary for this task.
The Trans-Atlantic Exoplanet Survey (TrES) has fol-
lowed the opposite approach: Instead of studying a few
selected fields with many faint stars, it monitors many
brighter stars using a whole network of small telescopes dis-
tributed over the globe. In 2004, the first planet, TrES-1,was
discovered by this program. It completes one orbit in 3 days
and has a mass of 0.6 Jupiter masses. Its radius is only slightly
larger than Jupiter’s: 1.08 Jupiter radii.
Table 4 lists the six extrasolar planets that were found so
far by the transit method, and Fig. 10 shows a comparison of
their radii and masses with the gas giants of the solar system.
The planet orbiting HD 209458 has still the largest radius
and lowest mean density of all transiting planets, despite the
fact the very hot Jupiters are even closer to their parent star.
The larger radius of this gas giant might be the consequence
of additional heating, possibly by tidal forces due to a slightly
noncircular orbit (although we will see later that this can be
ruled out).3.3.2 A MICROLENSING PLANET
In the summer of 2003, the two microlensing surveys,
OGLE and MOA (Microlensing Observations in Astro-
physics), independently detected a microlensing event to-
wards the galactic bulge. During the close monitoring of
this event, a strong 1 week long deviation from a single
lens lightcurve was discovered (see Fig. 11). Careful mod-
eling of the combined photometric data sets showed that
the lightcurve of the OGLE 2003-BLG-235/MOA 2003-
BLG-53 event is best described by a binary lens model with
an extremely small mass ratio of 0.004. In the probable case