Encyclopedia of the Solar System 2nd ed

894 Encyclopedia of the Solar System

TABLE 2 The first 8 Radial Velocity Planets

msini Orbital Period,

Star (Jupiter mass) P(days) a(AU) Eccentricity,e

51 Peg 0.5 4.2 0.05 0

70 Vir 7.4 117 0.48 0.4

47 UMa 2.5 1089 2.09 0.06

ρ1 Cancri 0.84 14.7 0.12 0.02

τBoo 4.13 3.3 0.05 0.01

υAnd 0.69 4.62 0.06 0.012

16 Cyg B 1.69 799 1.67 0.67

ρCrb 1.11 39.9 0.23 0.13

close proximity to the host star, these gas giant planets have
estimated upper atmosphere temperatures of more than
1000 K.

3.2.2 MORE RADIAL VELOCITY PLANETS

In the 2 years following the discovery of 51 Peg, astronomers
from the United States announced the discovery of 7 more
extrasolar planets orbiting Sun-like stars. All these detec-
tions were based on years of precise radial velocity measure-
ments of these stars using telescopes and spectrographs at
Lick, McDonald, and Whipple Observatories. Table 2 lists
the first 8 extrasolar planets discovered by the radial velocity
technique along with their orbital characteristics.

3.2.3 THE FIRST MULTIPLE PLANETARY SYSTEM

In 1999, the teams of the Lick and Whipple Observatory
Doppler surveys announced the discovery of the first extra-
solar multiplanetary system around a Sun-like star. The ra-
dial velocities ofυAndromedae deviated progressively from
the originally derived, single-planet velocity curve, and with
the additional years of data it became apparent that a triple
Keplerian model is required to describe the complex reflex

motion of this star. In addition to the previously found hot

Jupiter, this system contains two more giant planets with

msini= 1 .89 and 3.75 Jupiter masses at separations of 0.8

and 2.53 AU. Also their orbits have significantly nonzero ec-

centricities (0.28 and 0.27), making this system again quite

different from our solar system.

3.2.4 A TRANSITING PLANET

With the discoveries of more and more hot Jupiters, it was

just a matter of time until one of them would have a near

edge-on orbit so that the planet transits in front of the star.

The hot Jupiter companion to HD 209458 was the first

transiting extrasolar planet. The planet itself was first dis-

covered by the radial velocity method, but in this case the

photometric follow-up observations revealed—for the first

time—the characteristically shaped lightcurve of a transit-

ing planet (Fig. 5). With the viewing angle known (i= 86 ◦)

themsinivalue transformed into a true mass for the planet

of 0.69 Jupiter masses. The depth of the dip in the lightcurve

yielded a radius for the companion of 1.4 Jupiter radii.

With a known mass and a known radius, a mean density of

0.31 g cm−^3 was derived. This is an even lower mean density

FIGURE 5 The lightcurve of the star HD 209458 showing

the reduction in stellar flux by 1.5% due to the transit of its

hot Jupiter. From the depth of the transit lightcurve, a radius

of 1.4 Jupiter radii was derived, and combined with the

planetary mass—determined from radial velocities—a low

mean density of 0.31 g/cm^3 was found for this planet.