Beyond the Solar System:
Discovering extrasolar planets
Extrasolari Live! Project
27 February 2008
Powerpoint by G. Masi
“In some worlds there is no
Sun and Moon, in others they
are larger than in our world,
and in others more
numerous. In some parts
there are more worlds, in
others fewer (...); in some
parts they are arising, in
others failing. There are
some worlds devoid of living
creatures or plants or any
moisture ”.
Democritus ~460 - 370 a. C.
“There are infinite worlds
both like and unlike this world
of ours. For the atoms being
infinite in number, as was
already proven, (...) there
nowhere exists an obstacle to
the infinite number of worlds
”.
Epicurus 341 - 270 a. C.
“There cannot be more
worlds than one ”.
Aristotle 384 - 322 a. C.
“[…] This space we declare to
be infinite, since neither
reason, convenience,
possibility, sense-perception
nor nature assign to it a limit.
In it are an infinity of worlds of
the same kind as out own ”.
Giordano Bruno 1548 1600
In 1952, Otto Struve (1897 –
1963) mentioned both transits
and radial velocities as
techniques to spot exoplanets.
Previsions by Otto Struve
Velocità Radiali
(The Observatory, 72, 199-200 (1952)
Fenomeno
dei Transiti
Peter Van de Kamp (1901 - 1995)
A pioneer of the field, in 1963 he
proposed that around the “Barnard’s
star” there was a giant planet, explaining
its oscillating motion.
In addition to obvious statistical considerations, the existence of other
planetary systems is supported by our knowledge of their formation
mechanisms and the observation, around other suns, of circumstellar
disks, like the one in Beta Pictoris, discovered about 20 years ago.
There are no doubts about the existence of other planetary systems...
…but they are NOT easy to observe!
Why we are looking for them?
The discovery of exoplanets may help us addressing the questions
concerning the formation of planetary systems (including the one where
we live!) and understanding the meaning of life in the Universe.
A fascinating, but complex scenario, which helped new sciences to develop,
as esobiology.
We need many systems to have enough data for a statistical approach!
These researches will help us understanding what life
really is!
But…
Finding a extrasolar planet is a challenge for a number of reasons, at least
because:
- They are “small” (~ <10 Mj);
- They are “buried” in the light of their suns;
- Their distance do not help.
Astronomers need to look for side effects to spot them.
Nonetheless, in less than 15 years more than 270 exoplanets were found!
Never say never again…
While they are a new field in modern Astrophysics, exoplanets offer big
chances to amateur astronomers to do real science.
In several cases, amateurs contribuited to the discovery of extrasolar planets,
(as in the case of XO-2b!).
All this because a number of different techniques can be used to find them,
some of them accessible to amateur astronomers.
How to find them?
As the direct detection of exoplanets is currently a real challenge (there
are very few candidates observed by direct imaging), several other
techniques were developed to find planets around stars like our Sun,
(spectral classes F, G, K).
Three of them are quite powerful:
1) The study of radial velocities;
2) The observation of planetary transits;
3) The observation of gravitational microlensing effects.
The first technique is a spectroscopic one, requiring the decomposition of
the incoming light; the others are photometric ones, requiring the lightcurve
of the source.
Radial Velocity Technique
Studying the radial velocity of a given star, it is possible to observe
periodic oscillations due to the presence of a companion.
The amount of the oscillation depends on the orbital details, including the
orbit inclination along the line of sight, and on the mass ratio between the
two objects (assuming the simple case of only two bodies).
This approach takes benefits from the Doppler effect.
For example, Jupiter ‘produces’ on the Sun an effect of about 12 m/s.
The first star with a planet detected in this way was 51 Pegasi, similar to the
Sun
Rp
0.05 UA
P
4.2 giorni
MP
0.47-0.6 MJup
e
0
Limits of the Radial Velocity technique
With the current telescopes, radial velocities around 3 m/s can be
detected: they are not useful to search for Earth-like planets.
Mass estimates are just lower limits (because the orbit inclination is
involved).
It is possible to discover jovian-like planets, not very far from their star.
Transits
When the orbit makes possible to the planet to transit in front of its star as
seen from the Earth, then it is possible – in principle – to photometrically
spot it as a very small ‘eclipse’: a transit!
It is similar to the transit of Venus!
This technique has strong orbital constrains: rare events.
Transits
Simulation of the transit of a exoplanet.
The details of the lightcurve depend on the planetary ones.
If the planet is also observed by means of spectroscopy, then it is possible to
get very useful physical details, density included!
Transits: the good and the bad.
The size of the planet against the star determines the depth of the transit: a
Jupiter-like planet would produce a 1% drop of the light, photometrically
accessible with amateur-sized instruments.
It is possible to detect planets quite far from their star (however, more distant
the planet is, less probable the transit)
Small planets (Earth…) produce 0.01% large effects, not accesible to
ground based telescopes.
Gravitational microlensing.
Accordingly with General Relativity, the gravitational field deflects light,
working as a lens!
If between a star and the observer there is a massive boby, the light of the
star will experiment a gravitational lensing effect by the body in between. If
the latter has a stellar mass, then the deflected source is not splitted. If the
star is in motion, the observer sees a specific photometric evolution, which
can be properly modeled starting from the event geometry.
Generally, a peak in brightness is recorded: the peak amplitude depends on
the alignment between the star and the lens.
But some anomalies can occur, depending on the presence of planets
around the lens.
Gravitational microlensing: the good and the bad
It is sensible to Earth-size planets.
The search in concentrated in the galactic bulge, where the star density is
huge, to maximize the event probability.
Microlensing events are, by themselves, not-repeatable!!
Exoplanets at a glance
To date (20 feb 2008), 276 exoplanets are known.
We know 25 multiple systems.
Exoplanets detected by radial velocity are 260.
Transiting planets are 35.
Discoveries by microlensing are 6.
The planet with the largest mass known (with good confidence) is XO-3b:
13.24 jovian masses.
Gl 581c has the smallest mass (0.0158 jovian masses).
Exoplanets at a glance
Distribution by orbital
semi-major axis
The large number of bodies close
to their star is obvious: this is a bias
due to the observing techniques.
Exoplanets at a glance
Distribution by orbital
period
The sample is obviously dominated
by short-period planets.
Exoplanets at a glance
Distribution by minimum
mass
Distribution by orbital
eccentricity
Conclusions.
The known population of exoplanets is largely dominated by massive objects,
close to their star.
This is because of a bias in the sample due to the observing techniques.
Thanks to modern technology and promising space missions, we will be able
to “see” Earth-size worlds, placed in the habitability zone, (like Gliese 581c),
where water can exist at the liquid state.
Worlds able to host life, where we hope to find the answers to the big
questions of modern science.