Geoscience Reference
In-Depth Information
CM
x
Fig. 9.1
Radial velocity method cartoon. The
spectrum of the star gets blue-shifted and red-shifted
as the star moves towards and away from an
observer respectively.
Star and planet
moving around the
center of mass (CM).
the planet, this method of detection is sensitive
to the planetary mass
M
projected onto the line
of sight of the observer. This is the ''minimum''
mass of the planet (e.g.
M
sin
i
,where
i
is the an-
gle between the line of sight and normal to the
orbital plane). There is no signal for systems that
are face-on with respect to the Earth. Table 9.1
shows the RV effect that different companions
would have on their host star. The current de-
tection precision is at the level of 1 m/s, making
it possible to discover super-Earth in close-in or-
bits. The leap towards Earth analogs requires one
order of magnitude in improvement (10 cm/s), a
difficult but not impossible challenge. It entails
better resolving the spectral lines of the star com-
pared to some reference spectral lines. This is
obviously an engineering challenge of making the
most stable and accurate detector (i.e. specto-
graph). However, there is an intrinsic limitation
that arises from stellar noise (spots, flares, etc.),
which means that planets around active stars re-
quire much more data and analysis to uncover
them, if at all possible, from the stellar noise.
The second most successful method is the tran-
sit method, which exploits the fact that for favor-
able configurations the planet passes in front of
the star with respect to an observer, blocking part
of the light coming from the host star. This de-
crease in luminosity is proportional to the ratio of
the area of the planet to that of the star. Figure 9.2
shows a cartoon that depicts the principles of this
technique. With this method it is possible to mea-
sure the radius of the planet, if that of the star is
known, and learn about the configuration of the
system (the orbital inclination angle
i
), so that in
Secondary Transit:
starlight is partly
reflected and partly
thermally re-emitted
Primary Transit:
starlight passes
through the planet's atmosphere
(transmission spectrum)
Table 9.1
Radial velocity effect on the host star
induced by different companion planets.
Planet
During the primary
transit, the planet
blocks part of the
stellar light
Distance (AU)
Velocity (m/s)
Jupiter Mass
1
28
.
4
Time
Jupiter
5
12
.
7
Neptune Mass
0
.
1
4
.
8
Fig. 9.2
Transit method cartoon. As the planet moves
in front of the star (primary transit) it blocks part of the
stellar light. The dip in luminosity registered is
proportional to the ratio of the area of the planet to
that of the star.
Neptune Mass
1
1
.
5
Super Earth (5 M
E
)
0
.
1
1
.
4
Super-Earth (5 M
E
)
1
0
.
45
Earth
1
0
.
09
