Exoplanet plenitude
The main aim of the present paper is to give a brief overview of the revolution in exoplanet discoveries which started about two decades ago and the new concepts and perspectives that these observational findings have brought about. The level of the text is simple, as deemed suitable for reading by...
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irk-123456789-1191832017-06-05T03:04:01Z Exoplanet plenitude Martín, E.L. The main aim of the present paper is to give a brief overview of the revolution in exoplanet discoveries which started about two decades ago and the new concepts and perspectives that these observational findings have brought about. The level of the text is simple, as deemed suitable for reading by young scientists with different levels of expertise. The paper is organized in the following sections: 1) Historical background. 2) Basic concepts and definitions of what is a planet. 3) Observational evidence of planetary diversity and the theoretical pathways to explain what we see. 4) Future research directions. 2012 Article Exoplanet plenitude / E.L. Martín // Advances in Astronomy and Space Physics. — 2012. — Т. 2., вип. 2. — С. 109-113. — Бібліогр.: 83 назв. — англ. 2227-1481 http://dspace.nbuv.gov.ua/handle/123456789/119183 en Advances in Astronomy and Space Physics Головна астрономічна обсерваторія НАН України |
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The main aim of the present paper is to give a brief overview of the revolution in exoplanet discoveries which started about two decades ago and the new concepts and perspectives that these observational findings have brought about. The level of the text is simple, as deemed suitable for reading by young scientists with different levels of expertise. The paper is organized in the following sections: 1) Historical background. 2) Basic concepts and definitions of what is a planet. 3) Observational evidence of planetary diversity and the theoretical pathways to explain what we see. 4) Future research directions. |
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Martín, E.L. |
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Martín, E.L. Exoplanet plenitude Advances in Astronomy and Space Physics |
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Martín, E.L. |
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Martín, E.L. |
| title |
Exoplanet plenitude |
| title_short |
Exoplanet plenitude |
| title_full |
Exoplanet plenitude |
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Exoplanet plenitude |
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Exoplanet plenitude |
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exoplanet plenitude |
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Головна астрономічна обсерваторія НАН України |
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2012 |
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http://dspace.nbuv.gov.ua/handle/123456789/119183 |
| citation_txt |
Exoplanet plenitude / E.L. Martín // Advances in Astronomy and Space Physics. — 2012. — Т. 2., вип. 2. — С. 109-113. — Бібліогр.: 83 назв. — англ. |
| series |
Advances in Astronomy and Space Physics |
| work_keys_str_mv |
AT martinel exoplanetplenitude |
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2025-07-08T15:23:10Z |
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2025-07-08T15:23:10Z |
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1837092769497088000 |
| fulltext |
Exoplanet plenitude
E.L.Martín
∗
Advances in Astronomy and Space Physics, 2, 109-113 (2012)
© E. L.Martín, 2012
INTA-CSIC Centro de Astrobiologia, Carretera de Ajalvir km. 4, Torrejon de Ardoz, 28550 Madrid, Spain
The main aim of the present paper is to give a brief overview of the revolution in exoplanet discoveries which
started about two decades ago and the new concepts and perspectives that these observational �ndings have brought
about. The level of the text is simple, as deemed suitable for reading by young scientists with di�erent levels of
expertise. The paper is organized in the following sections: 1) Historical background. 2) Basic concepts and
de�nitions of what is a planet. 3) Observational evidence of planetary diversity and the theoretical pathways to
explain what we see. 4) Future research directions.
Key words: stars: brown dwarfs, stars: formation, planets and satellites: formation
historical background
On 24 August 2006, at the end of its 26th General
Assembly held in Prague, the International Astro-
nomical Union (IAU) passed a resolution on the def-
inition of the term �planet in the Solar System� which
made headlines around the world because it demoted
Pluto to the lesser category of dwarf planets. This
decision was adopted after a voting result of 237-137,
which implied that only a minority of the IAU mem-
bers actually casted a vote. The discovery of a few
objects beyond the orbit of Neptune with sizes com-
parable to Pluto (see e. g. [17]) prompted the IAU
to react. However, as noted by Nobel prize winner
Brian Schmidt in a public talk at the 220th meeting
of the American Astronomical Society in Anchorage
on June 12, 2012, the decision on Pluto is still con-
troversial. The statement of Prof. Schmidt on this
sensitive issue was �Once a planet, always a planet�.
Even though there is a clear de�nition, albeit con-
troversial it may be, for what a planet is in the So-
lar System, it is not straightforward to extrapolate
it to exoplanets. The masses of planets in the So-
lar System span about 4 orders of magnitude from
Jupiter to Mercury. This is one order of magnitude
more than the mass range for stars. Hence, the term
planet refers to a broad class of objects that includes
a wide range of properties.
Jupiter has mass of 1.89 · 1027 kg = 317.83 Earth
masses, which may seem as a lot, but in fact it is
a little less than 1/1000th of the mass of the Sun.
Just below 1/10th of a solar mass, electron degener-
acy sets in as the dominant pressure in the interior
and creates a kind of object named a brown dwarf
(BD) that behaves di�erently than stars do [30, 34].
Contrary to stars, BDs are not known to die and pro-
duce interstellar ashes; they rather keep cooling o�,
slowly shrinking and shining faintly forever. There is
no BD known in the Solar System, but other solar-
type stars are endowed with brown dwarf compan-
ions (see e. g. [59]). Nevertheless, most of the BDs
we know are single. They have been revealed mainly
by wide-area surveys (see e. g. [58]) and by pencil
beam deep surveys (see e. g. [39]). Very recently
cross-matching of large public catalogues using the
Virtual Observatory tools have demonstrated that
an increase in the e�ciency in the identi�cation of
BDs is possible [1].
The last two decades of the 20th century gave us
the discoveries of GD165B, a BD companion to a
white dwarf [8]; a planetary system around the pul-
sar PSR1257+12 [82]; PPL1 and Teide 1, bona �de
BD members in the Pleiades star cluster [61, 71];
Gl229B, a BD companion to a nearby star [51, 55],
and close-in substellar-mass objects around solar-
type main-sequence stars [36, 50, 44]. These �nd-
ings have galvanized planetary and stellar research
by providing the �rst examples of sub-stellar-mass
objects that do not exist in the Solar System. They
have also brought about much debate about the
meaning of the term �planet� and how to distinguish
it from the term �brown dwarf� [5, 14, 69, 73].
on the definition of �planet�
By analogy with the Solar System, an unambigu-
ous planet is an object that resembles one of the
eight major planets that orbit the Sun. However, it
is clear that such a narrow de�nition of what a planet
is has not been adopted in the astronomical litera-
ture. A pressing question is: how much is it useful
and reasonable to stretch the de�nition of what a
planet is? Clearly, there is no consensus in the com-
munity because drawing a hard boundary for what
a planet is, would include objects that are the fa-
vorites of some astronomers, but it may also exclude
objects that are the favorites of others if the de�-
nition is carefully crafted. For example, the planet
∗ege@cab.inta-csic.es
109
Advances in Astronomy and Space Physics E. L.Martín
de�nition proposed by the IAU Working Group on
Extrasolar Planets1 would include most of the plan-
ets discovered by the radial velocity technique, but
it would exclude those discovered around pulsars by
the timing method, and also the free-�oating (also
called nomads, rogue or unbound) planets found by
microlensing [74] and cluster planets found by deep
direct imaging in very young open clusters such as
Sigma Orionis and the Trapezium [42, 57, 83] and
follow-up spectroscopy [16, 47].
In the dim light of the still incipient understand-
ing of exoplanets, a simple, convenient and clear ap-
proach to a planet de�nition is not coming forward
yet. Several new kinds of objects have been discov-
ered in the last two decades that have some planetary
attributes and that do not exist in the Solar System
Table 1. This diversity reminds once again the an-
cient plenitude principle, discussed by great philoso-
phers dating back to Socrates and Plato, and includ-
ing Giordano Bruno and Immanuel Kant [28, 40],
and recognized as a general uni�cation theme in nat-
ural phenomena. A de�nition of what is a planet
should not fail to pay due respect to the diversity of
planets, which has been an astonishing revelation of
scienti�c endeavor in the last two decades, and con-
stitutes a modern veri�cation of the age-old principle
of plenitude.
Taking into account the currently available infor-
mation on the topics of BDs and exoplanets, an up-
dated de�nition of planet is o�ered here. It goes as
follows: A planet is an object that has a mass large
enough to have a gravitationally-dominated internal
structure, i.e. nearly spherical shape, but not large
enough to sustain any kind of stable nuclear fusion
over long periods of time. The word planet can be
used for any kind of celestial object that is, has been
or will be a planet.
By this de�nition, a planet can be found free-
�oating in space, unbound to a more massive object;
and a planet can also be gravitationally bound to
multiple stars, single stars, stellar remnants, failed
stars such as BDs or recycled stars such as blue strag-
glers. A planet can have smaller objects orbiting it
that are called satellites. When the mass ratio be-
tween a planet and a satellite is close to unity, the
system may be called a double planet.
From the cosmogony point of view, there can be
many di�erent pathways to form a planet, and this
casts some doubt that planet formation could be ei-
ther an adequate physical quantity or a useful obser-
vational criterion to de�ne what a planet is. In prac-
tice, it is convenient to adopt a planet de�nition that
heavily relies on the mass of the object because the
mass can be either measured directly or it has an im-
pact on observable quantities such as surface gravity.
For solar composition the boundary BDs and planets
is determined by deuterium fusion, which ceases to
be stable at around 13 Jupiter masses [18, 19]. Just
as the lithium test has e�ectively been applied as a
tool to distinguish between very low-mass stars and
BDs [6, 43, 46, 62, 72], the deuterium test has been
proposed to distinguish between BDs and planets [9]
but it has not been carried out yet because it is obser-
vationally very challenging. This important obser-
vational test may have to wait for the advent of the
30-meter class generation of ground-based telescopes
such as the European Extremely Large Telescope or
the American Thirty Meter Telescope. Particularly
promising targets are nearby late-T dwarfs with ef-
fective temperatures around 500K that appear to
have peculiar properties indicative of young age and
planetary mass, such as for example ULASJ1335+11
[37].
planetary plenitude
and formation pathways
The observational evidence for protoplanetary
disks around very young stars is overwhelming. One
of the most spectacular examples are the pictures
of photoevaporating disks obtained with the Hub-
ble Space Telescope near O-type stars in the Orion
star-formation complex [53]. Another impressive ex-
ample is the disk seen around the young (10Myr)
and nearby (19.5 pc) β Pic star [66, 75] which hosts a
massive planet (8MJup) at a semimajor axis of about
12AU [35]. Statistical studies of the infrared excess
among the stellar populations of star-forming regions
and young open clusters indicate that most stars
are born with surrounding circumstellar disks that
dissipate with a timescale of about 10Myr [4, 80].
However, while it is clear that planets form in cir-
cumstellar disks around newly formed stars, it is not
clear at all what are the properties of those planets.
None of the techniques that are widely used for exo-
planet detection works very well for very young stars.
So far the technique that has provided more exo-
planet detections at very young ages is direct imaging
from space or using adaptive optics from the ground
[20, 45], but it is limited to semi-major axis larger
than a few AU and only has sensitivity to planets
more massive than Jupiter.
In the near future a promising technique to iden-
tify exoplanets around newly formed stars is ac-
celerometry at near-infrared wavelengths using novel
calibration techniques such as new cells that combine
a variety of gas mixtures [2, 78, 79] or laser frequency
combs [54]. The pros and cons of di�erent calibra-
tion techniques are discussed in [49]. So far infrared
radial velocity has not revealed any new planet, but
it has been useful to rule out a giant planet around
one of the nearest TTauri stars (TWHya) which was
claimed by a study made at optical wavelengths [31].
Infrared radial velocities have also been used to re-
ject the presence of a planet around the nearby ul-
tracool dwarf VB10 [7, 65]. A multi-band approach
1http://www.dtm.ciw.edu/boss/iauindex.html
110
Advances in Astronomy and Space Physics E. L.Martín
to improve the RV precision in active stars has been
advocated by [12].
When all the known exoplanets are plotted in
mass versus separation diagram, the result is a scat-
ter diagram (see Fig. 1). Exoplanets are every-
where except where technical limitations preclude
detectability. Such a wide range of properties in-
dicate the presence of multi-parametric complexity.
The simple picture of planet formation as core accre-
tion of planetesimals in a disk around a single star
has been shattered by the extravaganza of exoplanet
discoveries. The solar nebula theory where the Solar
System stems out of a �attened disk of dust and gas
rotating around the Sun has been for over 200 years,
and still seems to be for some, the main conceptual
pillar of theories of planet formation [32, 33].
Fig. 1: Distribution of masses in Jovian units with re-
spect to semi-major axis in AU for 706 exoplanets from
the Extrasolar Planets Encyclopedia. Di�erent planet
formation pathways that are likely to populate di�erent
parts of the diagram are labeled.
It is now widely recognized that planets do not
necessarily stay where they were formed. Tidal in-
teractions of planets with their stars, interactions of
planets with the disks and gravitational scattering of
planets due to other planets, BDs or stars in multi-
ple systems can result in signi�cant orbital evolution
of newly born planets, and even produce free-�oating
planets via ejection. The large eccentricities of many
exoplanets are thought to indicate the in�uence of
planet-planet scattering [77] and the Kozai mecha-
nism [81]. Evidence for planet ejection may come
from observational studies of extremely young mul-
tiple systems such as the protobinary TMR-1 in the
Taurus-Auriga star-formation region [63, 76]. Both
planets and brown dwarfs are likely to form in mas-
sive disks [70], making it hard to distinguish these
two types of objects using some sort of cosmogonical
criteria.
future directions
in exoplanetary research
An obvious focal point of future research on ex-
oplanets is the detailed characterization of the myr-
iad of known systems, both the stellar hosts and
the planets themselves. Approved space missions
by ESA (GAIA and Euclid) and NASA (JWST)
will bring signi�cant advances in our understanding
of exoplanets and their stars. In analogy with the
brown dwarfs for which three new spectral classes
have been developed (L, T and Y), it has been sug-
gested to classify hot Jupiters as pL and pM class
[26]. It will be worthwhile to develop such analogies
even further and use brown dwarfs as benchmarks to
develop the tools needed for deriving fundamental
parameters of exoplanets such as chemical compo-
sition, e�ective temperature, rotational broadening
and surface gravity. A step in this direction is the
analysis of high-resolution near-infrared spectra of
late-M [22] and T dwarfs [21] using model atmo-
spheres. Prospects for spectroscopic characteriza-
tion of exoplanet atmospheres from space and from
the ground are indeed very promising [3] and may
be coupled with developments in high performance
coronography [29].
A somewhat more controversial issue is whether
signi�cant investments should be made in the direc-
tion of �nding Earth twins around solar-type stars.
The widely held views in the scienti�c community,
that the Earth does not occupy a special place in
the Universe, may not support the idea that a major
investment of public funds should be devoted to an
antropocentric strategy in exoplanet research. More-
over, it is clearly more cost e�ective to detect and
characterize habitable Earth-sized planets around
small primaries such as very low-mass stars, BDs and
white dwarfs [10, 11, 56]. Nevertheless, the urge to
�nd exoplanets as similar as possible to the Earth
around stars as similar as possible to the Sun is very
strong because of its connection to Earth-centered
branches of science such as Biology and Geology.
The existence of regular spacings in the orbits of
planets in the Solar System and among exoplanetary
systems is becoming well documented [13, 41]. This
long range order in planetary systems was �rst rec-
ognized as the Titius-Bode law. It is likely a feature
stemming out from the origins of the systems which
remains to be fully accounted for.
Consideration of planet formation and evolution
in binary systems opens new research perspectives.
From the theoretical point of view, the formation of
close-in planets in the excretion disks of merging bi-
naries should be explored in detail [48]. It is very
exciting to witness the detection of circumbinary
planets after over a decade of unsuccessful attempts
[23, 24, 25]). More e�orts devoted to revealing plan-
ets around post-binary evolution stars, such as blue
stragglers and R Corona Borealis stars, and stellar
111
Advances in Astronomy and Space Physics E. L.Martín
remnants will shed light on the processes of planet
formation and evolution in binary systems. In the
solar vicinity blue and red stragglers have been iden-
ti�ed which are likely the result of the coalescense
of stellar binaries [60, 67]. About 30 of them have
been identi�ed within 30 pc of the Sun [27, 64], and it
would be very worthwhile to increase their numbers.
The ubiquity of planets around stars implies that
they become excellent contenders to explain some
mysteries in stellar evolution. For example, HER-
SCHEL/HIFI observations of water vapor in AGB
stars [52] are not well understood. An intriguing
possibility that deserves further scrutiny is that the
water detected in those stars could be coming from
water-rich planets that spiral in toward the star dur-
ing the AGB phase. Some of the planets swallowed
by the stars during the course of post-main sequence
evolution may have an impact on the stellar mass loss
rate and rotation velocity [38]. Orbiting substellar-
mass companions have been posited as a plausible in-
gredient to model the non-axisymmetrical structure
of planetary nebulae [68]. A possible link between
lithium depletion, rotational history and the pres-
ence of exoplanets has been explored for solar-type
main-sequence stars [15].
Two decades after the discoveries of the �rst un-
ambiguous specimens, it has become very clear that
BDs and exoplanets are here to stay. They have pro-
vided a link between di�erent scienti�c communities
such as the geologists, the planetary scientists and
the stellar astrophysicists, and in the future they are
likely to foster much more interdisciplinary research.
acknowledgement
Funding for this research was provided by the
RoPACS FP7 RTN network and the CONSOLIDER-
INGENIO GTC project. Part of this research was
carried out during a visit to the Department of Ge-
ological Sciences of the University of Florida.
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Planetary Kind Mass Range Age Range Metallicity Range Known Number
Free Floating Planets 0.5 � 13 MJup 1�10 Gyr Solar a few dozens
Cluster Planets 3 � 13 MJup 1�10 Myr Solar a few dozens
Wide-orbit Planets (a>50) AU 6 � 13 MJup 1 � 100 Myr Solar 7
Close-in Planets (a<0.1) AU 0.2 MEarth � 8 MJup 10 Myr � 10 Gyr 0.3 � 3 × Solar 153
Super-Jupiter Planets 2 � 13 MJup 1 Myr - 10 Gyr 0.3 � 3 × Solar 213
Super-Earth Planets 1.5 � 10 MEarth 1 � 10 Gyr 0.5 � 3 × Solar 55
Pulsar Planets 0.02 MEarth � 2.5 MJup 1�12 Gyr unknown 5
White Dwarf Planets 0.37 � 7.7 MJup 1�12 Gyr unknown 11
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