Encyclopedia of the Solar System 2nd ed

Extrasolar Planets 899

TABLE 4 The Extrasolar Planets Discovered by Transit Searches

Planet Mass (Jupiter mass) Radius (Jupiter) Orbital Period,P(days)

OGLE-TR-56 1.45 1.23 1.21

OGLE-TR-113 1.35 1.08 1.43

OGLE-TR-132 1.19 1.13 1.69

OGLE-TR-10 0.57 1.24 3.1

OGLE-TR-111 0.53 1 4.02

TrES-1 0.61 1.08 3.03

that the primary lens is a normal star, the secondary lens
would be a∼1.5 Jupiter mass planet orbiting at∼3 AU.
Thus, this event is regarded as the first demonstration of the
discovery of an extrasolar planet by microlensing. A better
characterization of the star/planet system that caused this
lensing event has to await next-generation ground-based or
space telescopes, which will be powerful enough to resolve
the lens.

3.3.3 INFRARED RADIATION FROM EXTRASOLAR PLANETS

With the launch of NASA’s infrared space telescope
SPITZER, a new and very interesting spectral window for
observations of extrasolar planets became available: the
far infrared, where thermal emission dominates the ra-
diation coming from a planet (as shown in Fig. 3). Al-
though SPITZER lacks the spatial resolving power to detect
planets, its high sensitivity in the infrared can be used to dis-
cern between radiation from a star and its planet.
Two independent teams planned basically the same
SPITZER observations: to observe a transiting extrasolar

planet during (and out of) secondary eclipse (the time when

the planet is directly behind the star and hidden from view).

If the amount of infrared radiation measured by SPITZER

during the eclipse is less than outside the eclipse, then this

difference is the radiation coming from the planet itself.

This effect was indeed successfully measured for the two

transiting planets HD 209458 b and TrES-1 b. The amount

of planetary infrared radiation was used to estimate a “sur-

face” temperature for these two planets: The visible upper

atmosphere of HD 209458 b has a temperature of 1130±

150 K, and for TrES-1 b the respective value is 1060±50 K.

Both values are in good agreement with the expected tem-

perature of a giant planet heated by the intense irradiation

of the nearby host star. The exact timing of the secondary

eclipse of HD 209458 b also demonstrated that its orbit is

indeed circular and that tidal heating cannot be the expla-

nation for its abnormally large radius.

Interestingly (and quite ironically, because the informa-

tion is obtained by thelackof photons), these observations

also represent the first unambiguous detections of photons

emitted by extrasolar planets.

FIGURE 10 The radii and masses of transiting extrasolar

planets (HD 209456b, TrES-1, OGLE-TR-132b,

OGLE-TR-113b and OGLE-TR-56b) compared to Jupiter

and Saturn. The dashed lines are curves of constant density.

This shows that HD 209458b has an unusually low mean

density.