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Extrasolar Planets
to appear in Encyclopedia of Time, Sage Publishing, in preparation, H.J. Birx (Ed.)
The term extrasolar planets or exoplanets stands for planets outside our Solar System, i.e. not orbiting
the Sun, but other stars. Planets in our Solar System are defined as objects with enough mass to be
spherical and round by their own gravity and to be alone on their orbit around the Sun, i.e. to be the
dominant object in a particular orbit, and not to be a moon or asteroid (see the entry Planet in this
encyclopedia for the official definition, the historical debate, and a discussion of the planets of our
Solar System).
Most exoplanets are discovered by observing the stellar motion around the common center of mass of
the star+planet system, i.e. by observing somehow the motion of the objects in orbit around each other,
i.e. by measuring precisely the periodic variation of certain values, e.g. radial velocity or brightness,
with time, e.g. the first extrasolar planets were found with the timing technique around a pulsating
neutron star.
The recent definition of Planets of our Solar System by the International Astronomical Union deals
mainly with the question of the minimum mass for an object to qualify as planet and excludes Pluto.
This matter was raised by the fact that more and more objects similar to Pluto were discovered by
larger and larger telescopes. The questions of maximum mass and formation of planets were left out in
this new definition, possibly partly because there is not yet a consensus in the international community.
For a discussion of extrasolar planets, however, the maximum mass is very important, namely in order
to classify an object as planet or nor - and to distinguish between planets and brown dwarfs.
Both planets and brown dwarfs are sub-stellar objects in the sense that they are less massive than stars,
so that they cannot fuse normal hydrogen (as stars do to produce energy and to shine for a long time).
Brown dwarfs, while they cannot fuse normal hydrogen (atomic nucleus made up by just one proton),
can burn deuterium, i.e. heavy hydrogen (a proton plus a neutron), so that they are self-luminous for a
few millions of years until the original deuterium content is depleted.
The upper mass limit of planets can be defined either through the lower mass limit for deuterium
fusion, which is around 13 times the mass of Jupiter depending slightly on the chemical composition,
or by the mass range of the so-called brown dwarf desert (see next paragraphs).
We will next discuss the different exoplanet discovery techniques by chronological order of success
and thereby also discuss the properties of objects found so far.
Radial Velocity: Since a few thousand years, speculations exist as to whether other stars can have their
own planets. Both Giordano Bruno and Nikolaus of Cues have answered this question positively a few
hundred years ago. However, not until 1989, the first object was discovered which could really be an
extrasolar planets and which is today still regarded as planet candidate. This first extrasolar planet
candidate was discovered serendipitously by the so-called radial velocity technique: The velocity of the
motion of an object directly towards us or away from us (in one dimension) is called radial velocity and
can be measured by the so-called Doppler shift of spectral lines. Atoms in the atmosphere of stars can
absorb light coming from the interior of the star, namely at a certain energy, frequency, or wavelength
for each kind of atom or ion, producing absorption lines in the spectrum of the star. If the star moves
away from us, such lines are red-shifted (blue-shifted when approaching us), i.e. at a wavelength
different from the normal wavelength (larger for red-shifted). When a second object like a planet orbits
around a star, actually both objects orbit around their common center of mass. Hence, also the star
wobbles: It sometimes approaches us, sometimes flies away from us. This can be observed as
periodically changing radial velocity. The period of the variation gives the orbital period and the
amplitude of the change in radial velocity yields the mass of the companion. However, because the
inclination between the orbital plane and our line of sight is normally not known, only a lower mass
limit is known. Therefore, such low-mass companions detected by the radial velocity technique are to
be seen as planet candidates, they could have a mass above the maximum mass for planets, namely
then being brown dwarfs or even low-mass stars. The first such case was published in 1989 by Latham
et al., namely a companion with minimum mass of 11 Jupiter masses around the star HD 114762. This
planet is called HD 114762 b, always the first planet found around a star is called by the name of the
star plus a small b behind the star name (small letter c for next planet, etc). This planet may very well
have a true mass above 13 Jupiter masses, in which case it could be regarded as brown dwarf.
The first object discovered around a sun-like normal star, which is almost certainly a planet, is called
51 Peg b and is a planet with about half the mass of Jupiter as minimum mass found by Mayor &
Queloz around the star 51 Peg in 1995.
The radial velocity can nowadays be measured with an accuracy of about one meter per second, so that
planets with minimum mass of a few Earth masses can be detected by the wobble they produce on a
low-mass star. One Jupiter orbit, i.e. 12 years after the important discovery of 51 Peg b, there are now
about 250 planet candidates discovered. In some cases, several planet candidates are orbiting a single
star, in some other cases, individual planet candidates are found in binary stars. Because the high
precision of radial velocity technique is available since about the early 1990s, planets with more than
20 years of orbital period could not yet be discovered, one needs to observe at least one orbital period.
Many planet candidates found so far orbit their stars with only few days, which is quite different from
our Solar System, where the innermost planet Mercury needs several month to orbit the Sun. The high
number of planet candidates with short orbital period, however, can be seen as observational bias,
because such planets also introduce a larger wobble on their star due to Kepler´s and Newton´s laws of
gravity.
The mass range of all these planet candidates shows a strong peak at about one Jupiter mass and a
strong dip at around 20 to 30 Jupiter masses, even though this method would be biased towards more
massive companions, because they have a stronger effect on their central star. There are about 250
planet candidates all with masses below about 20 Jupiter masses, then almost no objects with minimum
mass between about 20 and 70 Jupiter masses, i.e. only few brown dwarfs, and then again a large
number of stellar companions (with minimum mass above 70 Jupiter masses). This paucity of brown
dwarfs from the radial velocity technique is called brown dwarf desert and the dip in the mass spectrum
is deepest at around 20 to 30 Jupiter masses. Either the lower mass range of the brown dwarf desert or
the minimum mass for deuterium burning can be used as upper mass limit for planets, if one would
define the upper mass limit for planets.
Pulsar timing: At the end of the life time of a massive star, after most of the material is burned by
fusion, the star collapses due to its own gravity, then forms a very dense and compact object made up
mostly by neutrons, called neutron star, while the rest explodes due to a rebound as super nova. A
neutron star typically has about 1.4 times the mass of our Sun, but a diameter of only 20 to 30 kilo
meters. Such neutron stars rotate very fast, sometimes even about 100 times per second, sometimes
once in few seconds. Most known neutron stars emit strong radio emission along their rotation axes
(beams), which appear pulsed due to the fast rotation. We should keep in mind that so-called Puslars
are not pulsating, but rotating fast. One can measure the rotation period as pulses with both high
precision and high accuracy, such objects are called Pulsars. In the case of the Pulsar called PSR1257,
Wolszczan & Frail discovered sinusoidal variations of the millisecond pulses in 1992 and interpreted
these variations to be caused by low-mass objects in orbit around the neutron star, with a mass of only
about the planet Earth. This discovery of Pulsar planets (by Pulsar timing) came as a big surprise,
because planets were not expected around neutron stars; it is still dubious as to whether planets can
survive the supernova explosion, or whether the objects found around PSR1257 are either remnants of
the explosion or formed afterwards.
Transit: In the case of planets or planet candidates discovered by the radial velocity technique, the
inclination of the orbit of the planet around the star in not known. One way to determine this inclination
would be a so-called transit, i.e. when the planet orbits around the star in our line-of-sight, so that the
planet moves in front of the star once per orbit (and behind the star also once per orbit). When the
planet is in front of the star (i.e. in front of the spatially unresolved stellar disk), a small part of the
stellar light is blocked by the planet. This event is called transit or eclipse. Such events also happen in
our Solar System, e.g. as seen from the Earth, the inner planets Mercury and Venus can path directly in
front of the Sun, which can even be observed spatially resolved. The transit light curve results in the
inclination of the orbit and also in the radii of star and planet. Then, one can determine not only the true
mass of the companion (planet or brown dwarf), but also from mass and radius its density. The first
case, where this was successfully observed was HD 209458 in the year 2000, the first radial velocity
planet candidate confirmed to be a true planet (and found to be a gas giant planet with low density like
Jupiter). About one Jupiter orbit after the discovery of 51 Peg b, there are about 33 transiting planets
known (November 2007), most of which have also been discovered first by the transit, then confirmed
as planets by radial velocity. In a few cases, also the secondary transit is detected, i.e. a small decrease
in the total combined brightness of star+planet (one should keep in mind that in all such cases, the
planet is not seen directly), when the planet is behind the star. From the difference in brightness
between the time of secondary transit and immediately before and/or after the transit, one can indirectly
determine the brightness of the planet.
Astrometry: While the radial velocity technique measures the wobble of the star due to the orbiting
planet in just one dimension (radial), one can measure the wobble in the two other dimensions by the
so-called astrometry, i.e. by very accurate and/or precise determination of the position of a star on the
sky. Our Sun as seen from about 30 light years distance also moves slightly on the sky due to Jupiter
orbiting the Sun, but this is a very small effect, less than one thousands of an arc second (the moon has
a diameter of 1800 arc seconds). The star GJ 876 was the first, where this wobble was detected, namely
with the Fine Guidance Sensor of the Hubble Space Telescope, confirming the radial velocity planet
candidate GJ 876 b to be a real planet with just about two Jupiter masses. In the meantime, a few more
planet candidates were confirmed by astrometry, and also one radial velocity planet candidate was
found to be a low-mass star. Other observing programs have started where one tries to discover such a
wobble also in stars, where no planets or candidates were found, yet, by other techniques, both from
ground-based as well as space-born telescopes.
Direct detection: All previous techniques, radial velocity, astrometry, and transits, cannot determine
which photons are coming from the stars and which photons from the planet, i.e. they cannot directly
detect (or see) the planet. While stars are bright and self-luminous due to fusion, planets are very faint,
mostly shining only due to reflected light, and they are also very close to their star, so that they cannot
be detected or seen next to their much brighter star. In case of young planets, which are still contracting
and/or accreting matter, they are self-luminous and, hence, not that faint, so that it could be less
difficult to directly detect a young planet next to a young star. Several observational campaigns were
started around the turn of the millenium in 2000 with the Hubble Space Telescope and ground-based 8
to 10 meter telescopes. In case of the ground-based observations, the Earth atmosphere is another
problem, leading to the twinkling of stars, so that we obtain images which do not have the best possible
images quality. With the new technique of so-called Adaptive Optics, one can de-twinkle the stars, i.e.
flatten the disturbed wave front with a deformable mirror in the telescope. With such a technique used
at the 8 meter Very Large Telescope of the European Southern Observatory in Chile, a few companions
to young stars were found since 2004, which could really be young planets detected directly, the first
such case was the star GQ Lupi with its companion GQ Lupi b detected by Neuhäuser et al. In such
cases, it is more difficult to be sure about the exact mass of the companion, because the orbital period is
several hundreds of years, so that these few objects could also be low-mass brown dwarfs.
Microlensing: According to Einstein's theory of general relativity, mass or matter deforms space, so
that a light ray moving close to matter would be diverted. One would see a ring of light (Einstein ring)
around the object. If such an Einstein ring is not resolved spatially due to small mass and/or small
angular resolution, one would still see the background object being brightened by the foreground
object, the gravitational lense. Such an event is called microlensing. If a binary lense (e.g. a star plus a
planet) would move – as seen from us – directly in front of a background single star, one would
observe a double-peaked light curve, one brightening due to the star and one brightening due to the
planet. This way, one can detect planets also in large distances of thousands of light years. There are a
few cases, where such a double-peaked light curve has been observed, which could possibly be due to a
planet. However, in all such cases, due to the large distance and, hence, small brightness, the nature,
mass, and distance of neither the lense (or the primary objects in the double lense, the star) nor of the
lensed background object is known, so that the mass of the secondary object in the lense (possibly a
planet) cannot be determined without large uncertaintly. The mass of the companion is determined from
the mass of the primary (unknown) and the brightness ratio of the two peaks. Practically, such events
can never be observed again nor confirmed.
Timing of pulsating stars: Some stars towards the end of the normal life time, i.e. after most of the
light material is burned, are pulsating, i.e. periodically increase and decrease their volume and, hence,
brightness. Such a pulsation is observable as periodic brightness change. With precise observations, one
can detect a small periodic variation in the pulsations, which can be explained by a wobble of the star
due to an orbiting low-mass object, very similar to the Pulsar timing and radial velocity technique. In
the case of the pulsating star V391 Peg, such a variation was detected recently, which can best be
explained by a planet candidate with 3 Jupiter masses as minimum mass. This star has burned all its
hydrogen already, has expanded to the red giant phase, has lost large amounts of its material, and is
now again contracted to become a so-called White Dwarf. It is the first time, that a planet candidate is
detected in a star after the red-giant phase, except the Pulsar planets. This case shows, that planets can
survive the red giant phase. Our Sun will undergo this red giant phase in a few billions of years, it will
then expand very much to swallow Mercury and Venus. It is not clear, yet, as to what effects this will
have on the Earth, but it is likely that all life will be extinguished.
All these different techniques to discover planets have resulted in several hundred planets and planet
candidates, including some planetary systems, where several planets orbit the same star (updates on
planet discoveries can be found on www.exoplanets.orgwww.exoplanet.eu or ). Planetary systems
consist not only of the planets and their host star, but also of minor bodies like asteroids, comets, and
moons, but often if not always also of a circumstellar disk with dust remaining from the formation
phase. This is also the case in the Solar System, where dust in the so-called zodiacal disk can be
observed in dark moon-less nights due to reflection of Sun light on dust particles; such dust debris
disks can also observed around other stars, with or without planets.
All the planets discovered so far have masses of at least several Earth masses and are much different
from the Earth. It is not yet possible to detect Earth-like planets. Such discoveries may be possible in
the future either by larger telescopes or by ground- or space-based interferometry, i.e. the combination
of several telesopes.
Another eminent question is habitability of exoplanets. So far, no signs of life have been found on other
planets, neither in our Solar System nor elsewhere. It is difficult not only to define life, but also to
detect Earth-like planets – not to speak about Earth-like or even non-Earth-like life on distant planets.
However, it may well be that life could form either on some already detected planets or on their moons,
if existing.
Ralph Neuhäuser
Astrophysikalisches Institut und Universitäts-Sternwarte
Friedrich-Schiller-Universität Jena
Schillergässchen 2-3
D-07745 Jena
Email: [email protected]
Literature cited:
Latham D.W., Stefanik R.P., Mazeh T., Mayor M., Burki G., 1989, The unseen companion of
HD114762 - A probable brown dwarf. Nature 339, L38
Mayor M. & Queloz D., 1995, A Jupiter-Mass Companion to a Solar-Type Star. Nature 378, 355
Neuhäuser R., Guenther E.W., Wuchterl G., Mugrauer M., Bedalov A., Hauschildt P.H., 2005,
Evidence for a co-moving sub-stellar companion of GQ Lup. Astronomy and Astrophysics 435, L13
Wolszczan A. & Frail D., 1992, A planetary system around the millisecond Pulsar PSR1257+12. Nature
355, L145
Literature usefull for further reading:
Boss A.,
Looking for Earths: Race to Find New Solar System
1998
Clark S.,
Extrasolar Planets
Wiley 1998
Cole G.H.A.
Wandering Stars
Imperial Colleges Press 2006
Dvorak R.,
Extrasolar Planets: Formation, Detection, and Dynamics
Wiley 2007
Mason J.,
Exoplanets, Detection, Formation, Properties, Habitability
Springer 2007