Encyclopedia of the Solar System 2nd ed

Extrasolar Planets 893

3. Observations of Extrasolar Planets

3.1 The Pulsar Planets

The first discovery of planets outside our solar system was
achieved by Alexander Wolszczan and colleagues in the
early 1990s. This discovery would set the tone for all sub-
sequent discoveries of extrasolar planets in terms of their
strangeness. Wolszczan found the planets orbiting the mil-
lisecond pulsar PSR B1257+12 using the Arecibo radio
telescope and the timing method described in Section 2.
The first surprise was that the planets orbit a “dead” star,
which had undergone a previous supernova explosion. It
is unlikely that these planets existed before the star went
supernova.
The more plausible scenario is that these planets some-
how formed after the explosion. Millisecond pulsars are
believed to achieve their high rotation rates due to spin
up by in-falling material accreted from a companion star.
The planets might have formed during this process in the
accretion disk around the pulsar.
The planetary companions to PSR B1257+12 are also
remarkable in a different way: they have very small masses.
Due to the extreme sensitivity of the timing method in the
case of millisecond pulsars (where the arrival time of a pulse
can be measured with microsecond precision), even com-
panions with the mass of our Moon or less can be detected.
Table 1 summarizes the properties of the PSR B1257+ 12
planetary system. Soon after the system was discovered, the
mutual gravitational perturbations of the planets on each
other were measured, thus confirming that they are indeed
planets and not a previously unknown effect, intrinsic to the
pulsar.
So far these companions represent the lowest mass ob-
jects known to orbit a star other than the Sun. In terms
of mass these planets are also the most Earth-like extra-
solar planets we know. However they must be barren and
dead worlds because of the constant bombardment by high
energy radiation coming from the pulsar.
Just recently Wolszczan presented evidence for a fourth
object with a mass of only 15% the mass of Pluto orbiting
the pulsar at a distance of 2.7 AU. This new object, however,
qualifies as an asteroid or comet rather than a planet.

TABLE 1 The PSR B1257+12 Planetary System

Msini Orbital Period,

Planet (Earth mass) P(days) a(AU)

A 0.015 25.34 0.19

B 3.400 66.54 0.36

C 2.8 98.22 0.47

3.2 Planets Around Sun-like Stars: The Success of

the Radial Velocity Technique

3.2.1 51 PEGASI: THE FIRST PLANET ORBITING A SOLAR-TYPE

STAR

In the fall of 1995, two Swiss astronomers, Michel Mayor

and Didier Queloz, stunned the community as well as the

public by their announcement of the discovery of the first

extrasolar planet around a sun-like star. Their precise ra-

dial velocity measurements of the star 51 Pegasi revealed a

periodic variation of 4.2 days and an amplitude consistent

with anmsini= 0 .5 Jupiter mass companion (Fig. 4). The

minimum mass of the object firmly places this companion

into the gas giant planet mass range.

However, the extremely short orbital period and small

orbital separation of 0.05 AU were surprising in many ways,

and alternative explanations for the 51 Peg radial velocity

signal were put forward. Stars more evolved than the Sun

show similar variability, which is caused by pulsations rather

than by Keplerian motion. But 51 Peg passed every test for

this type of variability, and soon Mayor and Queloz’s claim

of having found the very first planet orbiting a “normal” star

was generally accepted.

51 Peg b represents the prototype of a new class of plan-

ets that soon emerged from the results of the radial veloc-

ity surveys, the so-calledhot Jupiters. Because of their

FIGURE 4 The radial velocity measurements (dots with error

bars) of the solar-type star 51 Pegasi phased to the orbital period

of its planetary companion. The sinusoidal variation is caused by

a companion withmsini= 0 .5 Jupiter masses in a circular orbit

witha= 0 .05 AU and an orbital period of 4.2 days. Reproduced

with permission fromNature.