Beyond the Solar System:

Discovering extrasolar planets

Extrasolari Live! Project27 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.

Fenomeno dei Transiti

Velocità RadialiPrevisions by Otto Struve

(The Observatory, 72, 199-200 (1952)

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!

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.

Why we are looking for them?

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!

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!

But…

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!).

Never say never again…

All this because a number of different techniques can be used to find them, some of them accessible to amateur astronomers.

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.

How to find them?

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.

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.

Radial Velocity Technique

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

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.

Limits of the Radial Velocity technique

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!

Transits

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

The large number of bodies close to their star is obvious: this is a bias due to the observing techniques.

Distribution by orbital semi-major axis

Exoplanets at a glance

The sample is obviously dominated by short-period planets.

Distribution by orbital period

Exoplanets at a glance

Distribution by orbital eccentricity

Distribution by minimum mass

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.