Download The first cool rocky/icy exoplanet

Dominik, Horne, Bode: Extrasolar planet
The first cool
rocky/icy
exoplanet
1: Artist’s impression (by
Herbert Zodet, ESO) of
OGLE-2005-BLG-390Lb with
its (unseen) red dwarf star
OGLE-2005-BLG-390L in the
background. An atmosphere
surrounds the icy surface.
This picture is available
as a computer desktop
background from http://www.
eso.org/outreach/press-rel/
pr-2006/phot-03-06.html.
Are we really alone? The discovery of the cool, rocky and icy planet OGLE-2005-BLG-390Lb using the
technique of gravitational microlensing suggests that we are not. Martin Dominik, Keith Horne and
Michael Bode explain the technique and how this discovery changes our view of the universe.
Abstract
The recent discovery of the first cool,
rocky/icy exoplanet around a mainsequence star provides the first
observational hint that planets like Earth
are common in the universe. It was
detected using gravitational microlensing
from data of three independent observing
teams under the lead of PLANET
(Probing Lensing Anomalies NETwork),
operating a common microlensing
campaign with the UK-based RoboNet
project. In the coming years intense
microlensing observations will determine
the abundance of cool rocky/icy planets
around M-dwarfs and therefore provide
the first observational test of models of
planet formation and migration on objects
that share their evolutionary history with
Earth and other planets on which life had
a chance to develop.
A&G • June 2006 • Vol. 47 O
n 26 January 2006, three independent
campaigns announced the discovery of
OGLE-2005-BLG-390Lb, the first cool,
rocky/icy planet orbiting a main-sequence star
other than the Sun (Beaulieu et al. 2006). This
discovery from gravitational micro­lensing, the
technique of monitoring stars in the galactic bulge
magnified by the bending of light due to the gravitational field of an intervening foreground star,
provides the first observational hint that planets
like Earth are common in the universe.
Until about a decade ago, all our knowledge
about planets was based on the nine planets
of our solar system – an incomplete sample, to
say the least. These gave us a picture of small
rocky inner planets (Mercury, Venus, Earth,
Mars), outer gas giants (Jupiter, Saturn, Uranus,
Neptune), and the odd ice planet (Pluto). Our
understanding of planet formation was forced
to undergo a revision with the detection of the
first planets around main-sequence stars other
than the Sun (Mayor and Queloz 1995). These
revealed their existence by means of Doppler
shifts in the spectra of nearby stars, caused by
the gravitational pull of the orbiting planets,
creating a wiggle in the radial velocity of its
host star. But because these planets had to be
large to cause an observable Doppler shift, the
planetary systems revealed were unlike the solar
system; astronomers were faced with massive gas
giants in close orbits around their host star. By
January 2006, about 170 extrasolar planets were
known, of which more than 90% were found in
radial-velocity surveys whereas 6 were detected
from partial occultations of their parent star
while they pass in front of it (see figure 2). The
method of observing the periodic displacement
of a potential planet’s host star has also been
suggested, but due to insufficient astrometric
accuracy this technique has not provided any
detections yet.
First, make your planet
The currently favoured scenario for planet formation is that of core-accretion, where in a first step
planetary cores form from condensed mat­erial in
3.25
Dominik, Horne, Bode: Extrasolar planet
orbit period (for solar-mass stars)
30 d
1 yr
10 yr
3d
100 yr
FU
N
Doppler: 160
ET
/R
ob
oN
et,
µ
104
PL
AN
1000
, VLT
10
0.1
0.01
F
1
I
SIM
S, HAT
WASP, TrE
OG
LE
,
Keck
MO
A,
100
MP
planet mass (Earths)
the protoplanetary disc around their host star.
While in an inner hotter zone only grains of dust,
sand and pebbles clump together, planet formation is further supported by the presence of icy
snowballs in a cooler zone outside the so-called
“ice boundary”. Planets forming there are likely
to grow to gas giants by accreting hydrogen and
helium, and to migrate inwards, with a fair fraction ending up in close orbits: these constitute
the hot planets. The expected distribution of
planets as a function of their mass and orbital
axis depends critically on the mass of their host
star. For different stellar masses between 0.2 M⊙
and 1.5 M⊙, figure 3 shows the results of simulations carried out by Ida and Lin 2005. One
can distinguish at least three different classes of
planets: hot planets that have been dragged into
close orbits; cooler giants that formed from icy
cores in the outer regions and migrated inwards;
and cool rocky and icy planets. For the region in
which radial-velocity surveys are sensitive, these
simulations are in remarkable agreement with the
observed distribution of semi-major axes of orbits
and planet masses. While gas giants are predicted
to be much less abundant for less massive stars
(also in agreement with the observational data
from radial-velocity surveys), a strong population of cool, rocky/icy planets is predicted for
any mass of host star, which includes the Earth
and other planets with the potential to support
life. However, different techniques are required
to study planets of this evolutionary type. The
range of orbital semi-major axes between 0.1
and 10 AU appears to be a good match with the
sensitivity region of gravitational microlensing,
and this technique currently is the only one capable of detecting planets with mass as low as that
of Earth.
OGLE-2005-BLG-390Lb
Kepler
transits: 6+3
0.01
astrometry: 0
0.1
1
orbit radius (AU)
microlensing: 3
10
100
2: Distribution of known extrasolar planets with mass and orbital axis or period (for a solar-mass
host star) as well as the detection limits for various techniques: radial velocity/Doppler wobble
(black) for velocities of 3 m s–1 and 1 m s–1; astrometry (green) for ground-based (Keck, VLTI) and
space-based observations (SIM); eclipsing transits (blue) from the ground (WASP, TrES, HAT) or
space (Kepler); and microlensing (red) ground-based (PLANET/RoboNet, OGLE, MOA, µFUN) or
space-based (MPF) campaigns. The planets of our solar system are marked by yellow circles. While
the larger velocity and the larger range of inclinations for which an eclipse can occur favour closer
orbits for the radial-velocity and the transit technique, respectively, the astrometric signal increases
with orbit size. For all these techniques, a few orbits need to be observed, so the duration of the
campaign limits the orbit size for which planets can be detected. This restriction does not apply for
microlensing, making it the only technique capable of detecting planets more distant than ~8 AU from
their parent star within a reasonable time. Microlensing is most sensitive to orbital radii around the
Einstein radius rE = DL θE ~ 2–4 AU, where the detection limits are of statistical nature. The only principal
limitation arises from the finite size of the source star washing out the signal for planets below a
critical mass, which is around the mass of the Earth for ground-based campaigns, while the higher
photometric accuracy that can be achieved from space would even allow the detection of martianmass planets. The unique capability of microlensing for detecting low-mass planets at several AU,
and the fact that it is currently the only technique that can detect Earth-mass planets, are reflected by
the unique position of OGLE-2005-BLG-390Lb in discovery space among all known exoplanets.
Gravitational microlensing
The phenomenon of bending of light by the gravitational field of stars, now commonly known
as “gravitational microlensing”, was noted by
Einstein as early as 1912, but he did not publish
the corresponding “little calculation” in full until
much later (Einstein 1936) after intense persuasion by Czech engineer and amateur scientist R
W Mandl. For those interested in the historical details, there is a delightful article by Renn,
Sauer and Stachel (Renn et al. 1997).
According to General Relativity (Einstein
1915), a point-like (spherically symmetric and
non-rotating) object with mass M bends a light
ray passing at a distance ξ by an angle α̂ = 2RS/ξ
(for ξ ≫ RS), where RS = (2GM)/c2 denotes the
Schwarzschild radius; G is the universal gravitational constant, and c is the vacuum speed of
light. For this gravitational lens, in contrast to
the usual shaped lenses made of material with
larger refractive index than the surroundings,
the deflection angle decreases with distance from
the centre. The deflection law implies that for an
observed background star at distance DS, per3.26
fectly aligned with an intervening foreground
star with mass M at distance DL, a ring-like
image of the size of the angular Einstein radius
___________
√
DS – DL
​ ​
θE = ​ 2RS ​ _______
DL – DS
(1)
is created. This ring breaks up into two distorted
images of the source star, separated by ~2θE, if
the intervening “lens” star is slightly displaced
from the line-of-sight. For source stars in the
galactic bulge, typical distances are DS ~ 8.5 kpc
and DL ~ 6.5 kpc, for which
θE = 600 (M / M⊙)1/2 µas
(2)
This means that current optical telescopes cannot resolve these images. However, the distortion
of the images results in an observable magnification of the source star given by
2
u_____
+ 2 ​ A(u) = ​ ________
u​√u2 + 4 ​ (3)
for the lens and source star being separated by
u θE. With µ denoting (the absolute value of) their
relative proper motion, the source moves by θE
relative to the lens within tE = θE /µ and for uniform motion, one finds
___________
√ ( )
t – t0 2
u(t) = ​ u02 +​ ​ _____
​ ​ ​
t E
(4)
where u0 θE is the closest angular approach,
occurring at epoch t0. A typical proper motion
µ ~ 15 µas d–1, implies a microlensing event timescale
tE ~ 40 (M / M⊙)1/2 days
(5)
i.e. around a month for lens stars between 0.3 M⊙
and 1 M⊙.
Einstein concluded that “there is no great
chance of observing this phenomenon”. In order
to maximize the chances, it appears most promising to monitor dense resolved fields of stars
providing as many target stars as possible, such
as the Magellanic Clouds or the galactic bulge.
This chance was quantified by Petrou (1981) and
Paczyński (1986) for stars in the Large Magellanic Cloud and by Kiraga and Paczyński (1994)
for stars in the galactic bulge. They found that,
in each case, only about one in a million stars is
A&G • June 2006 • Vol. 47
Dominik, Horne, Bode: Extrasolar planet
4 (a): The PLANET (Probing Lensing
Anomalies NETwork) telescopes.
Their distribution in longitude
allows round-the-clock
coverage during the
galactic bulge observing
season from May to
September each
year.
(b): Locations of
RoboNet sites.
Boyden (1.5 m)
SAAO (1.0 m)
(a)
103
102
101
1
103
102
101
Mp (Earth mass)
1
Perth (0.6 m)
Canopus (1.0 m)
103
102
101
1
103
102
La Silla Danish (1.54 m)
101
1
(b)
103
102
101
1
10–1
1
a (AU)
101
3: Distribution of planets around host
stars with different mass resulting from a
simulation based on core-accretion models
(Ida and Lin 2005). The migration of planets
has artificially been halted at 0.04 AU.
Separated by population gaps, one can easily
distinguish hot planets, cooler gas giants and
cool rocky/icy planets. While planets below
~10 M⊕ between 0.1 and 10 AU are predicted
to be common for all masses of the host star,
gas giants become rare for the lower masses
resembling M-dwarfs. (Courtesy of S Ida)
brightened by more than 30% at a given time
– no great chance, indeed. But unlike in 1936,
technological advances in the development of
computers and digital cameras have made surveys
involving frequent monitoring of tens of millions
of stars feasible. Consequently, the first microlensing surveys directed at the Magellanic Clouds
and the galactic bulge went into operation from
1990, and the first event was reported in 1993 by
the MACHO (MAssive Compact Halo Object)
collaboration (Alcock et al. 1993). Nowadays,
the OGLE (Optical Gravitational Lensing Experiment) collaboration, using the 1.3 m Warsaw
Telescope at Las Campanas Observatory (Chile),
and the MOA (Microlensing Observation in
Astrophysics) collaboration, using their 1.8 m
telescope at Mt John Observatory (New Zealand), monitor more than 100 million stars daily.
Online data analysis allows them to issue public
alerts on about 1000 microlensing events per year
A&G • June 2006 • Vol. 47 while they are in progress (Udalski 2003). These
events are designated with the name of the survey
campaign, the year, “BLG” for the galactic bulge
and a running number, so that, for example, the
147th event alerted by OGLE in 2003 is named
OGLE-2003-BLG-147.
Mao and Paczyński (1991) showed that planets
orbiting the lens star can reveal their existence by
causing an observable deviation to the symmetric
light curve that would be created by a single isolated lens star. While a deviation of the desired
amplitude is less likely for less massive planets,
only the finite size of the source star limits it.
With 1% photometric accuracy achievable from
ground-based telescopes, planets with masses as
small as that of Earth can be detected, and even
planets with the mass of Mars become detectable with a space-based mission. With q denoting the planet-to-star mass ratio, the duration
of the planetary signal is of the order of the time​
__
√ q ​t E, during which the source moves by the
angular Einstein radius of the planet, or the time
t⋆ = θ⋆ / θE, in which it moves by its own angular
radius, whichever is the larger. This equates to a
range from a few hours to a few days, which in
any case is much smaller than the orbital period,
so that a snapshot of the planet is obtained. The
only parameters characterizing the planet that
affect the light curve therefore are the mass ratio
q and a separation parameter d, where d θE is the
instantaneous angular separation from its parent
star. While this means that very little information about the orbit is provided, in particular its
inclination, eccentricity and orbital phase are
unknown, it allows the detection of planets in
wide orbits which is otherwise prevented because
the orbital period would exceed the lifetime of a
project – and/or its investigator!
The largest sensitivity is reached when the planet’s angular separation from its host star is around
3.27
Dominik, Horne, Bode: Extrasolar planet
the angular Einstein radius θE, which for a lens
star at DL ~ 6.5 kpc with a mass between 0.3 M⊙
and 1 M⊙ gives a projected distance rE ~ 2–4 AU.
It is a lucky coincidence that the gravitational
radius RS of stars (a few km) combines with typical distances within the Milky Way (few kpc) to
result in a distance that is comparable to reasonable planetary orbits (few AU).
The daily sampling of the microlensing surveys appears not to be sufficient for characterizing planetary anomalies in the light curves.
Public alerts on events as observed and online
provision of photometric data, however, allows
other teams to carry out follow-up observations. The PLANET (Probing Lensing Anomalies NETwork) collaboration uses a network
of 1 m-class optical telescopes (shown in figure
4a) distributed in longitude around the southern hemisphere in order to allow round-theclock coverage of galactic bulge microlensing
events during the observing season from May
to September each year. Having started a pilot
campaign in 1995, PLANET now involves 33
collaborators affiliated with 19 institutions in
10 countries (France, UK, Denmark, Germany,
Austria, Chile, Australia, New Zealand, USA,
South Africa), and is led by Jean-Philippe Beaulieu (Institut d’Astrophysique de Paris, France)
and Martin Dominik (University of St Andrews,
United Kingdom). Its objective is to achieve a
photometric accuracy of 1–2% and a standard
sampling interval of between 1.5 and 2.5 hours,
which can be decreased to a few minutes for
ongoing anomalies (Dominik et al. 2002).
Since 2005, PLANET has been operating a common microlensing campaign with
RoboNet-1.0 (http://www.astro.livjm.ac.uk/
RoboNet). Led by astronomers from Liverpool
John Moores University on behalf of a consortium of 10 UK universities including St Andrews,
RoboNet-1.0 is a PPARC-funded prototype global network of the largest fully robotic (2 m) telescopes built to date (figure 4b). It comprises the
Liverpool Telescope (http://telescope.livjm.ac.uk;
sited on La Palma, Canary Islands, Spain) which
is a national facility of the UK for time-domain
astronomy (figure 5), plus time acquired for the
project from the Faulkes Telescope North (Maui,
Hawaii, USA) and Faulkes Telescope South (Siding Spring, Australia). All three telescopes were
designed and built in Merseyside by Telescope
Technologies Ltd, taking advantage of expertise within the Astrophysics Research Institute
of LJMU, and are controlled from the ARI’s
Telescope Management Centre. From October 2005, ownership of the Faulkes Telescopes
passed from the Faulkes Telescope Corporation
to the Las Cumbres Observatory. Besides searching for planets through micro­lensing (Bergdorf
et al. 2006), RoboNet also follows up gammaray bursts as its other core science programme
– ­science which is again critically dependent on
rapid and flexible response.
3.28
5: The Liverpool Telescope in its fully open enclosure at the Roque de los Muchachos Observatory on
La Palma (Canary Islands), 2400 m above sea level. (Dr R Smith, Liverpool John Moores University)
Virtual astronomers
With the smaller field-of-view of the telescopes
used by PLANET/RoboNet (compared to the
OGLE and MOA survey telescopes), the smaller
diameters of some of these, and the aim of highprecision nearly continuous coverage, it is not
possible to monitor all events that the surveys
pinpoint. Some alerts are immediately disqualified by being too faint or in too crowded fields.
We aim to select our targets and their sampling rate so that the detection efficiency for
planetary companions is maximized. In order
to achieve this goal, the current properties of
all ongoing events are monitored and the most
promising targets selected following a prioritization algorithm. Except for the Perth 0.6 m
telescope, which is automated, the observing
priorities are passed to an observer at each of
the PLANET telescopes who then follows the
instructions. For the RoboNet telescopes, however, the observing requests are submitted to the
sophisticated eSTAR software (http://www.estar.
org.uk), which was developed in a collaboration between Exeter University and LJMU. This
new technology allows a network of telescopes
to make detections and to respond much faster
than any human. User agents working as “virtual
astronomers” collect, analyse and interpret data
24 hours a day, every day of the year, alerting
us only when a discovery is made. Regularly
updated light curves of monitored events as well
as their parameters are available on the PLANET
web pages (http://planet.iap.fr). Moreover, further information about ongoing identified or
suspected anomalies is provided there. Major
updates are also provided as emails to anybody
who has subscribed to our PLANET anomaly
alert mailing list.
Because microlensing relies on the chance
alignment of unobserved foreground lens stars
with observed background source stars, there
is very little choice in the type and distance of
the former; indeed, an indication of the nature
of the foreground star is only obtainable during
the course of the event by measuring the event
timescale tE. The mass density and kinematics of
the galaxy dictates that the lens stars, and therefore the planets orbiting them, lie at distances
between 4.5 and 9 kpc. With the probability for a
lens star___
to cause a microlensing event increasing
with ​√M ​, about 50% of the events are caused
by M-dwarfs, whereas only 20% involve G-stars
(like the Sun) or earlier types. For a source star
located in Baade’s window (l,b) = (1°, –3.9°), the
probability density for the mass of the lens star
is shown in figure 6.
The absence of planetary signals in 42 events
well-covered by PLANET observations from
1997 until 1999, gave the first significant upper
limits on the abundance of gas giants around
M-dwarfs: it was found that less than 1/3 of the
stars that gave rise to the monitored events host
Jupiter-mass planets at orbital radii between
1.5 and 4 AU (Albrow et al. 2001). But it took
until 2003 for the first planet to be discovered
through microlensing, orbiting the lens star that
caused event OGLE-2003-BLG-235, also known
as MOA-2003-BLG-53 (Bond et al. 2004). With
a mass about 1.5 times that of Jupiter, the planet
created a strong signal in the form of spikes a
week apart, which could be detected by the
OGLE and MOA surveys. A further Jupiterlike planet, about three times as massive, was
detected in April 2005 in event OGLE-2005BLG-071, using data from OGLE, µFUN, MOA
and PLANET/RoboNet (Udalski et al. 2005).
The event OGLE-2005-BLG-390 was reported
by the OGLE Early-Warning System (EWS) on
A&G • June 2006 • Vol. 47
Dominik, Horne, Bode: Extrasolar planet
1
100%
galactic bulge
(Baade’s window)
(l,b) = (1°, –39°)
DS = 8.5 kpc
Γ–1 dΓ/d(lg M /M�)
75%
50%
0.5
25%
M
0
0.01
K
G
F
A
B
O
0%
0.1
1
M /M�
10
6: Probability density for the mass of the lens causing a microlensing event towards the galactic
bulge on a source star located in Baade’s window (l,b) = (1°, –3.9°) at DS = 8.5 kpc, as given by its
differential contribution to the event rate Γ. It is assumed that the lens is a main-sequence star,
where the spectral type and approximate colour are indicated, or a brown dwarf. The bold line shows
the cumulative distribution. About 50% of the events are caused by M-dwarfs, while just 20% involve
G-stars or earlier types.
11 July 2005, and was subsequently monitored
with the telescopes constituting the PLANET/
RoboNet network. Figure 7 shows the field
around OGLE-2005-BLG-390 as observed with
the Danish 1.54 m telescope by PLANET. From
the reddening-corrected luminosity I0 = 14.25
and colour indices (V–I)0 = 0.85 and (V–K)0 = 1.9,
the source star appears to be a G2–4 giant with
angular radius θ⋆ = (5.25 ± 0.73) µas, corresponding to R⋆ = (9.6±1.3) R⊙ at DS = 8.5 kpc. The
light curve, involving data from six different
observing sites, is shown in figure 8. It first followed the characteristic brightening expected
for a single isolated lens star and a point-like
source star, and reached a peak magnification
of about 3 on 31 July. On 10 August, however,
an anomalous rise of 0.15 mag was observed by
PLANET/RoboNet with the Danish 1.54 m telescope at ESO La Silla (this was the night starting on 9 August in Chile, but the early hours of
10 August in the UK). An OGLE point from this
night showed the same trend. By monitoring the
second half of this anomaly, which lasted about
a day, with the Perth, West Australia, 0.6 m telescope, and because we had previously obtained
a dense coverage of the peak region of the event,
we were able to conclude that the lens star has
a low-mass planet in orbit. It inherited its name
OGLE-2005-BLG-390Lb because it is a planetary secondary (“b”) to the lens star (“L”) that
caused the 390th microlensing event towards
the galactic bulge (BLG) alerted by OGLE.
The MOA collaboration was able to identify
the source star on its frames and confirmed the
observed deviation.
From the light curve, we inferred an event
timescale tE = 11.0 d, in which the source star
moves by one angular Einstein radius θE relative
to the lens star on the sky, making OGLE-2005A&G • June 2006 • Vol. 47 BLG-390 one of the shorter events. For this
bright and large source star, its size significantly
affects the light curve during the planetary deviation, which allowed us to determine t⋆ = 0.28 d,
in which the source moves by its own angular
radius θ⋆. This yields a proper motion
µ = θ⋆/ t⋆ ~ 20 µas d–1 = 7.6 mas yr–1
and the angular Einstein radius θE = µtE ~ 210 µas.
The two extractable parameters characterizing
the planet are the mass ratio q = 7.6 × 10–5 and
d = 1.610, where d θE is the current angular separation from its parent star. Mass and distance of
the lens star are related by the measured angular
Einstein radius, but neither of these quantities
is obtained directly. However, we were able
to calculate probability densities based on the
likelihood that a certain value would produce
the observed event with its given parameters,
assuming mass functions for lens and source
stars in the galactic bulge and disc as well as
their spatial mass density and velocity distribution for a double-exponential disc and a tilted
barred bulge (Dominik 2006). The results of this
analysis, shown in figure 9, yield the masses of
lens star and planet as M ~ 0.22 M⊙ (2.1) and
m ~ 5.5 M⊕ (2.1), respectively, and the lens distance DL = (0.85 ± 0.15)RGC, where RGC ~ 7.6 kpc
denotes the galactic centre distance. Assuming
circular orbits and averaging over the unknown
orbital orientation and phase, we found an
orbital radius a = 2.9 AU (1.6) and a period
P = 10.4 yr (2.0). Except for the lens distance, the
logarithmic average is quoted and numbers in
brackets indicate uncertainty factors corresponding to the standard deviation of the logarithm
of the respective quantity. We have also found
a 95% probability for the source being a mainsequence star rather than a stellar remnant, and
an 80% probability that the lens resides in the
7: True-colour image of the field around OGLE2005-BLG-390 resulting from a combination
of images taken by PLANET in three different
filters (BVI) at the Danish 1.54 m telescope at
ESO La Silla (Chile). (Courtesy of the PLANET
collaboration)
galactic bulge as compared to a 20% probability
that it belongs to the galactic disc.
OGLE-2005-BLG-390Lb appears not to be
massive enough to accrete enough gas to grow
into a gas giant planet like Jupiter or Saturn.
Instead, its expected surface temperature of
50–70 K points to an icy nature, making it a
more massive version of Pluto or resembling the
cores of the ice giants Uranus or Neptune, rather
than the inner rocky planets like Venus or Earth.
An artist’s impression of the planet, created by
ESO, is shown in figure 1. With lens and source
separating at a rate µ = 7.6 mas yr–1, cutting-edge
instruments could in principle allow the resolution of the lens star from the source star and
measurement of its magnitude in the foreseeable
future. Such measurements would result in a reliable (20% uncertainty) determination of its mass
(and thereby that of planet OGLE-2005-BLG390Lb). The biggest challenge here is with the
contrast between lens and source magnitudes,
which is expected to range between 7 and 8.5
for the K-band, while for shorter wavelengths it
becomes even larger, between 9.5 and 12 for V.
The detection of a planet with five Earth
masses by microlensing opens a new window
on the discovery of planets. For the first time,
a rocky/icy planet that is in an orbit resembling that of the Earth around the Sun has been
detected. This gives the first experimental hint
that planets of this type, which includes all planets that provide conditions suitable for life to
develop, are common.
How common are these planets?
Radial-velocity surveys indicate that the fractional abundance of giant planets around Mdwarfs is about 10 times smaller than around
F- or G-stars, indicating fJ ~ 1–2%, which seems
3.29
Dominik, Horne, Bode: Extrasolar planet
1.6
3
3
magnification
2.5
1.5
OGLE
2
1.4
1
1.3
2000
3000
planetary
deviation
3592
3593
2
1.5
OGLE
Danish
RoboNet
Perth
Canopus
MOA
1
3560
3580
JD – 2450000
3600
8: Planetary model light curve of event OGLE-2005-BLG-390, along with data collected by PLANET/
RoboNet with the Danish 1.54 m telescope at ESO La Silla (Chile), the Perth 0.6 m (Western Australia),
the Canopus 1.0 m (near Hobart, Tasmania) and the Faulkes Telescope North 2.0 m (Haleakala,
Hawaii), as well as by OGLE (Las Campanas, Chile) and MOA (Mt John, New Zealand). Also shown are
model light curves for an isolated single lens star without planets (orange, short-dashed), and for a
binary source star (light black, long-dashed), which fail to match the observed data.
0.5
0
3
p (DL/RGC)
tE = 11.0 d
t� = 0.28 d
–5
q = 7.6×10
25%
1
m/M⊕
0%
10
75%
50%
1
25%
0.6
0.8
DL/RGC
1
0%
1.2
100%
(c) orbital radius
d = 1.610
2.5
2
1.5
1
0.5
0
75%
50%
1
0
100%
(b) lens distance
2
0
0.4
p (lg a/[1 AU])
1
0.1
(a) mass
p (lg P/[1 yr])
p (lg M/Mref)
1.5
DS = (1.05 ± 0.25) RGC, (l,b ) = (359.73°, –2.36°)
M/M�
1
100%
3
θ� =
75%
(5.25±0.73) µas
2
50%
25%
1
a/(1 AU)
0%
10
100%
(d) period
75%
50%
25%
10
P/(1 yr)
100
0%
9: Properties of the planetary system that caused the event OGLE-2005-BLG-390: mass of the planet
m and its host star M, their distance DL as well as the orbital radius a and period P. The four panels
show the probability densities of lg m/M⊕, lg M/M⊙, DL/RGC, lg a/(1 AU) and lg P/(1 yr), where RGC ~ 7.6 kpc
denotes the distance to the galactic centre, together with their cumulative distribution (in bold).
Constrained by the measured event timescale tE and the proper motion µ = θ⋆/t⋆, these are based on
the assumption of a double-exponential disc and a tilted, barred bulge as well as a mass function for
the lens stars (Dominik 2006), while stellar typing provides a prior estimate for the distance of the
source star DS = (1.05 ± 0.25) RGC. For deriving the orbital radius and the period, randomly oriented
circular orbits have been assumed, where period, mass and orbital radius are related by Kepler’s
third law. While the dots on the abscissa and the arrows mark the expectation value and the standard
deviation, the vertical line indicates the median and the shaded region a central 68.3% confidence
+5.5
+0.21
+1.5
+8.7
+0.12
interval. The latter give m = 5.5–2.7
M⊕, M = 0.22–0.11
M⊙, a = 2.6–0.6
AU, P = 9.0–2.9
yr, and DL = 0.86–0.14
RGC.
to agree with the small number of detections
by microlensing. A study of the detection efficiencies for the PLANET campaign suggests
that terrestrial planets are about 15 times less
likely to be detected than massive gas giants, so
that the respective number of detections points
to an abundance ratio fE / fJ ~ 10 and therefore
fE ~ 20%. The current PLANET/RoboNet telescope network is expected to yield ~50–80 fJ
3.30
detections of massive gas giants and ~5–10 fE
terrestrial planets within three years. This lets us
expect to find a few more cool, rocky/icy planets
around M-dwarfs over the coming years, allowing a measurement of the relative abundance
of gas giants and rocky/icy planets, and in turn
providing the first observational test of models of
planet formation and migration on objects that
share their evolutionary history with Earth and
other planets suitable for harbouring life.
The deployment of further 2 m-class robotic
telescopes at two additional prospective sites in
the southern hemisphere (for example, in Chile
and South Africa) would significantly boost
the planet detection capabilities to ~200–250 fJ
giants and ~20–25 fE terrestrial planets. The
enhanced number of detections would then also
allow us to study the distribution of properties
(mass and orbital axis) of planets made of rock
and ice. ESA’s Darwin mission and NASA’s TPF
(Terrestrial Planet Finder) both aim to look for
signs of life on planets around nearby stars by
determining the abundance of biomarkers such
as oxygen, ozone, methane and nitrous oxide as
well as the greenhouse gases water vapour and
carbon dioxide in their atmospheres. These missions are planned to be deployed from 2015 and
will cost several billion pounds. Prior to that, one
would like to know whether there are a sufficient
number of promising candidate planets to look
at. At this time, gravitational microlensing is the
only way to find out.
The discovery and formation of extrasolar
planets forms one of the research initiatives of the
recently formed Scottish Universities Physics Alliance (SUPA), allowing Scotland to build an international profile in the interdisciplinary emerging
field of astrobiology. In a cooperation between
the universities of Edinburgh, St Andrews and
Strathclyde, researchers aim to understand the
origin and diversity of life on Earth, in the solar
system, and throughout the universe. Besides
theoretical studies of star and planet formation,
the University of St Andrews has established a
strong profile in the hunt for extrasolar planets
involving different techniques. This not only
includes a leading role for the planet-hunting
strategy and the interpretation of the results of
the PLANET/RoboNet microlensing campaign,
but also involvement in the SuperWASP project
(Pollacco 2005), a wide-angle search for eclipsing transits, and the study of young stellar objects
and debris discs (Greaves 2005). ●
References
Albrow M D et al. 2001 ApJ 556 L113.
Alcock C et al. 1993 Nature 365 621.
Beaulieu J-P et al. 2006 Nature 439 437.
Bond I A et al. 2004 ApJ 606 L155.
Burgdorf M J, Bramich D M, Dominik M, Bode M F,
Horne K D and Steele I A 2006 Exoplanet Detection via
Microlensing with RoboNet-1.0 to appear in P&SS.
Dominik M et al. 2002 P&SS 50 299.
Dominik M 2006 MNRAS 367 669.
Einstein A 1915 Sitzungsber. preuss. Akad. Wiss. 47 831.
Einstein A 1936 Science 84 506.
Greaves J S 2005 Science 307 68.
Ida S, Lin D N C 2005 ApJ 626 1045.
Kiraga M, Paczyński B 1994 ApJ 430 L101.
Mao S, Paczyński B 1991 ApJ 374 L37.
Mayor M, Queloz D 1995 Nature 378 355.
Paczyński B 1986 ApJ 304 1.
Petrou M 1981 PhD thesis, University of Cambridge.
Pollacco D 2005 A&G 46 19.
Renn J, Sauer T and Stachel J 1997 Science 275 184.
Udalski A et al. 2005 ApJ 628 L109.
Udalski A 2003 Acta Astronomica 53 291.
A&G • June 2006 • Vol. 47