For intelligent life to exist in the Universe, other solar systems with
planets that have relatively stable environments must exist. While there are strong theoretical
reasons that to believe that solar system/planetary formation is a natural
consequence of star formation, it wasn’t until 1995 that first bonafide extra
solar planetary system was discovered. The details of that discovery come below. Since 1995 (and as of July 7, 2008), 304
new individual planets have been discovered around nearby solar type
stars. This rapid and successful
rate of planetary detection has lead to really a world wide community effort to
find solar systems like our own that have the highest probability of harboring
earth like planets. This
effort will eventually culminate in the terrestrial planet
finder mission of NASA – one of its most important missions ever although
this mission is still at least a decade away from realization.
As mentioned before, planetary formation will occur after there has been
sufficient time for the Galaxy to be seeded with heavy elements created through
the process of stellar evolution and supernova ejection. Thus we expect to find planetary systems
around stars that have been created within the last few billion years because
in that case the proto-stellar cloud will have been enriched in heavy elements.
This expectation is clearly borne out by the data: (from Fischer and Valenti, Astrophysical Journal 2005,
622,1102 – in public domain)
Consider the case of our Sun and Jupiter. Jupiter is sufficiently massive
that it actually exerts a small but noticeable gravitational tug on the Sun. To an external observer, the Sun will be
observed to have a small change in its radial velocity. Radial velocity refers to the motion
that is either directly towards or away from an observer. Motion of an emitting source
towards or away from an observer will change the wavelength/frequency of the radiation
received by the observer relative to that emitted by the source. This is called the Doppler Effect or
Doppler Shift. The
Doppler shift can be easily explained via the diagrams below:
In the top diagram, the fire engine is not moving but nonetheless testing
its siren for all to enjoy. In
this case, the frequency that is detected (by your ear) is the same as the
frequency that is emitted.
But what happens when the fire engine is moving, from left to right, as
illustrated in the bottom diagram.
In this case, the sound waves are compressed in the direction of motion
as the fire engine comes towards (blueshift) observer A – that observer hears a
higher pitched siren. In the case
of Observer B, the fire engine is moving away from them, along their direct
line of site (e.g. radial motion) and the waves are elongated in this case (as
the source is moving away from the observer - redshift) and the observer hears
a lower frequency siren.
The fire engine system is completely analogous to the case where a planet
and its host star are in mutual orbit about their center of mass; As shown below, at certain times in the
orbit the star will be moving directly towards the observer (blueshift) and at
other times the star is moving directly away from the observer (redshift):
Animated Version:
The amplitude (peak to trough variation) of this curve depends upon the
mass and separation from the star of the perturbing planet. The period of the
curve is the orbital period of the planet. In this particular case, the orbital period is about 3 years
and many orbital periods have been detected, thus giving confidence of a real
planetary detection. The vertical
lines going through each data point represent the precision of that measurement
– note that the post 1994 measurements all have much higher precision than the
earlier ones.
Therefore, if we can monitor nearby stars for radial velocity variations
at the level of a few meters per second, then we can potentially detect Jupiter
mass planets in orbit about them. Until
recently, however, this was easier said than done. To produce a detection,
precision radial velocity observations are needed. Until about 1995, this was
not possible with instrumentation. For instance, the typical precision was 25
meters per second, and such an instrument clearly would not detect a system
like Jupiter and our Sun. In
addition, one generally needs to observe a full orbital cycle to be sure that
you have detected the presence of a perturbing planet. So, for instance, even an external
observer was monitoring the radial velocity variation of our Sun over a 6 month
period, they would not be able to detect the presence of Jupiter, since its
orbital period is 5 years.
The race to find new planets officially began in October 1995 with the
discovery of a Jupiter mass planet in the star 51 Pegasus. The details of that discovery, as well
as an early example of using the Internet as a way to communicate scientific
discoveries to the public, can be found at the original site: http://zebu.uoregon.edu/51peg.html (which has remained unchanged since its
inception).
Given the success achieved to date, it’s fairly clear that the excitement
of the 51 Peg system has infused the astronomical community with a systematic
and determined effort to find and characterize as many extra solar planetary
systems as possible. As the
precision of our measurements gets better, the ability to find lower mass
planets increases and a recently completed survey down a the European Southern
Observatory in Chile has successfully found 5 new planets around nearby, very
low mass stars, of masses 4 to 25 times than of the Earth. These low mass planets were discovered
primarily because they are at distances equivalent to the distance between
Mercury (0.1 Earth masses) and the Sun, so the amplitude of their perturbation
was fairly large.
For example, Jupiter has a mass of 316 Earth masses and is located at a
distance of 5 AU from the sun (1 AU = 1 Astronomical Unit = distance from the
Earth to the sun). The
amplitude of the radial velocity variation on the host star essentially
directly with the mass and also with the square root of the distance between the
host star and the planet. For the
case of the Jupiter-Sun system, the more exact perturbation in radial velocity
of the sun is 12.7 meters/second. From that reference point, one can
tabulate the following combinations – this will help give you a feel for doing
the interactive exercise on the Doppler Wobble Method. For example, if you moved Jupiter twice as close, from 5 AU to 2.5 AU the
velocity perturbation on the Sun would increase by the square root of 2 to an amplitude of 12.7 * √2 = 18
meters/second.
Configuration |
Planet Mass (Jupiter Masses) |
Separation (in AU) |
Amplitude (meters/second) |
Period (years) |
1 |
1.0 |
5.0 |
12.7 |
11.1 |
2 |
0.1 |
1.0 |
2.8 |
1.0 |
3 |
15 Earth Mass |
0.1 |
9.0 |
0.03 |
4 |
1 Earth Mass |
0.5 |
0.13 |
0.35 |
5 |
1.0 |
20.0 |
6.4 |
89.4 |
6 |
3.0 |
20 |
19.1 |
89.4 |
Referring now to the above table:
·
If your instrumental configuration were limited to 10 meters/second then
only configurations 1 and 6 could be reliably detected.
·
There is no chance of detecting configuration 4 with current
instrumentation.
·
10-20 Earth Mass planets can only be detected if they are within 0.1-0.2
AU from the host star (the orbital distance of Mercury from the Sun is 0.1 AU)
·
Configuration 2 is barely detectable with the best ground based
instrumentation.
·
Configurations 5 and 6 would require decades of observation before
detected. Generally speaking you
need to able to detect at least ¼ of an orbit with reliable velocity changes to
have secure detection.
Therefore, the kinds of extra solar planetary systems that can be
detected depend upon a) the instrumental precision and b) the time period of
the observations relative to the intrinsic orbital period of the
variation. As a result,
certain kinds of configurations are more easily detected than other kinds of
configurations. But this doesn’t
mean that those other kinds of configurations don’t exist.