Transcript Number of planets

PLANETARY RESEARCH TEAM
EXOPLANETS TRANSIT
SEARCH THE SKY!
ASSOCIAZIONE ASTROFILI
CRAB NEBULA
COELUM ASTRONOMIA
Detection techniques - future possibilities
Angelo Angeletti
Tolentino (MC), ITALY – 31 October, 2007
Extrasolar planets
– A bit of history
– Detection techniques
– Present results
– Our observations
– Future work
A bit of history
What is a planet?
The term “planet” stems from a Greek word meaning ‘wanderer’. All
celestial objects moving across the sky were dubbed ‘planets’, as
opposed to the ‘fixed stars’. The list included the Sun, the Moon,
Mercury, Venus, Mars, Jupiter and Saturn – the only ‘planets’ visible to
the naked eye.
On 24 August, 2005, the International Astronomical Union (IAU)
defined ‘planet’ every celestial body which:
- orbits around the Sun;
- is massive enough to attain a spherical shape;
- has swept the region of its orbit clean of all the debris
A bit of history
The Solar System
A bit of history
How did the Solar System form?
The Sun and planets are believed to
have formed from a contracting
nebula of interstellar gas; the
process took place about 4.6 billion
years ago
According to modern theories, the
primordial nebula was mainly
composed of hydrogen and helium
(though heavier elements and solid
grains were also present) and it
must have been very cold (10 K)
The Orion nebula. This gas cloud hosts the cradle
of a number of stars.
A bit of history
How did the Solar System form?
About 4.55 billion years ago, due to
self-gravity, the density at the
center of the primordial nebula
grew steadily; further contraction
gave birth to our Sun.
The process increased at the same
time the rotation velocity as well as
the centrifugal force. The outer
parts of the nebula flattened to a
disk, while still rotating around the
newly-formed star.
Protoplanetary
disks (proplyds)
in the Orion
nebula
A bit of history
How did the Solar System form?
During the final stages of the collapse a strong stellar wind must have set
in, dragging the lighter elements outward – mostly hydrogen and helium.
As the temperature of proto-Sun rose
high enough to ignite thermo-nuclear
reactions, some bodies inside the disk
began to grow by collision and
gravitational capture processes,
sweeping their zone clean from debris.
This led to the formation of the protoplanets, from which the present
planets originated.
The proto-Sun became a yellow mainsequence star.
A bit of history
How did the Solar System form?
The Working Group on Extrasolar Planets (WGESP) of the IAU defines
as an extrasolar planet (shortened exoplanet) “…a body whose mass lies
below the threshold value for the onset of deuterium thermo-nuclear
fusion (which is about 13 Jupiter masses [MJ] for a typical solar
composition) and at the same time is orbiting a star or a star’s remnant no matter how evolved.
The minimum mass required is Mercury‘s - not Pluto!
Bodies less massive than 70 MJ (but still above the 13 MJ threshold) are
to be considered ‘brown dwarfs’ – no matter how they formed.
Free bodies (as those found in young stellar clusters) with masses below
the 13 MJ limit are brown subdwarfs, not planets.
A bit of history
Why search for extrasolar planets?
The search for exoplanets is a most recent field in Astronomy. Being
strictly related to a number of “hot” topics in other cultural areas – such
as religion, philosophy and much more else – it is increasingly becoming
an issue of paramount importance.
In due time – perhaps earlier than we might expect! – it may even give
an answer to a crucial question in the history of mankind...
Do other life forms and other inhabited worlds exist?
A bit of history
The beginnings
In the past, the existence of extrasolar planets
was reputed a plausible scenario. The first
scientific discussions about the issue date back
to 17th century.
Sir Isaac Newton was the first modern scientist
to give credit to the existence of exoplanets
(1713).
Supposedly confirmed reports of exoplanets’
‘discoveries’ abounded in 19th century.
IsaacNewton
A bit of history
The beginnings
A brand new research field opened up in 1984, when
a circumstellar disk around the star β Pictoris was detected.
A bit of history
The beginnings
Several discovery reports were issued in the following years.
1989: Latham detects a 10 MJ body circling the star HD 114762.
1991: Alexander Wolszczan identifies two planets about the same
mass as Earth’s, revolving around a pulsar (PSR 1257+12).
1993: Gordon Walker claims that oscillations in radial velocity of the
star γ Cephei might be caused by a ≈ 2 MJ planet.
However, such findings were considered too “weird” by most
scientists to be taken very seriously.
A bit of history
The beginnings
On 6 October, 1995, in Florence, the discovery of a planet near the star
51 Pegasi is announced. This star is 50 light years away, and very similar
to our Sun.
The mass of the planet is about 160 terrestrial masses (0.5 MJ) ; it orbits
very close to its star (7.5 million kilometers), in about 4 days.
The discoverers: Michel Mayor and
Didier Queloz, of the Geneva Observatory
An artist’s impression of 51 Pegasi
A bit of history
The beginnings
The date of 6 October 1995 marks the beginning of a thorough, extensive
search for extrasolar planets.
At 30 October 2007, 260 exoplanets in 224 stellar systems had been
discovered, located as follows:
198 single systems
18 double systems
6 triple systems
2 quadruple systems
Detection techniques
The various methods
Most exoplanets haven’t been
actually seen through a telescope
Direct observation of an
exoplanet is an exceedingly
difficult task. Its light is usually
much fainter (a millionth or even
less) than its parent star’s.
2M1207 b – one of the four extrasolar planets
discovered through direct observation
Detection techniques
The various methods
Apart from a direct observation, several methods for detecting
exoplanets have been developed. These are:
– The astrometrical method
– The radial velocity method
– The transit method
– The gravitational microlensing method
– The timing method
Detection techniques
The astrometrical method
The position of the star is
measured with the highest
possible accuracy, with the
purpose to detect a displacement
- however slight caused by a planet (both bodies
orbit around the center of mass).
For comparison, Jupiter – when
seen from a distance of 10 light
years – makes our Sun oscillate
of about 1 millionth of grade,
with a period of about 12 years.
Detection techniques
The astrometrical method
By this technique only very massive
pianets – and very close to their star –
can be detected: the so-called hot
Jupiters.
The mass of a hot Jupiter is the same as
Jupiter’s or even more, but it revolves
around the parent star at a distance less
than 0.05 AU (7,5 million km), which is
eight times closer than Mercury to our
Sun.
The typical temperature on the dayside
can reach a thousand degrees Celsius.
An artist’s impression of HD 209458b. The
blue tail is the planet’s atmosphere,
evaporating into space due to the close
proximity of the parent star.
Detection techniques
The radial velocity method
The gravity of a planet close to its star induces small variations in the
star’s radial velocity (i.e., the velocity along the line connecting Earth
and the star). Such variations can be detected in the star’s spectrum, by
measuring the shift of the spectral lines. This gives information about
the planet’s mass and orbiting distance.
Line shifts are
very small and
are
proportional to
the planet’s
mass.
Detection techniques
The radial velocity method
This method has given the best results so far.
From the collected data, and making use of Kepler’s laws,
some fundamental properties can be deduced – namely, the
orbital period, the distance from the parent star, plus an
estimate of the planet’s mass (the last parameter depending on
the orbit’s inclination as seen by the observer)
Detection techniques
The transit method
A planet crossing the disc of its parent star
(in so performing a transit), it causes a
small eclipse; the star’s brightness drops
then slightly.
In order to be able to detect a transit, two
conditions are to be met:
- Earth, planet and parent star must be
sharply aligned (this seldom happens);
- Observations must take place just when
the alignment is achieved.
Detection techniques
The transit method
Detection techniques
The transit method
Only hot Jupiters have been
detected during transits so far plus, they had all been
previously discovered with
radial velocity measurements.
On 30 October, 2007 only 29
planets (out of a total
reckoning of 260) had been
seen transiting over the disc of
their parent star.
Detection techniques
The transit method
By observing a transit the actual
size of a planet can be estimated.
The next step is spectral analysis.
This means taking two spectra one as the planet crosses the star’s
disc, the other when it passes
behind and it’s eclipsed by the star.
By subtracting the spectra one can
then get useful information about
the planet’s atmosphere.
Another artist’s impression of HD 209458b.
Detection techniques
The gravitational microlensing method
This method makes use of a passing star intercepting the light path from
another star that’s much farther away. If both stars are aligned with
respect to Earth, the gravity of the nearer bends the light rays coming
from the farther (gravitational lens effect), enhancing its luminosity for a
short time. If the passing star hosts a planet, a second luminosity peak can
be observed.
Detection techniques
The timing method
A rotating pulsar (the small, ultra-dense remnant of an exploded
supernova) emits radio waves at very regular intervals.
The timing method consists in measuring any changes in these time
intervals.
Slight anomalies in the time intervals can be used to detect changes
in the pulsar’s motion, which may be caused by one or more nearby
planets.
The results
At the date 30 October, 2007 260 planets are known:
– by using the radial velocity method, 247 planets orbiting 213 stars
have been discovered. 25 stars host a multiple system; a total of 29
planets transit over their star’s disc;
– the gravitational microlensing method has revealed 4 planets
revolving around 4 stars;
– 4 planets orbiting 4 stars have been discovered by direct
observation;
– the timing method has revealed 5 planets revolving around 3 stelle
(one of which hosts a triple system).
Present results
Some exoplanets
Gliese 876 b – The first planet detected around a red dwarf (Gliese
876). It orbits nearer to its star than Mercury does around the Sun.
HD 209458 b – First observed transit of an exoplanet over the disc
of its parent star; it also marked the first detection of an exoplanet’s
atmosphere.
Upsilon Andromedae – The first detection of a multiple planetary
system; it is composed by three planets, all Jupiter-type giants.
Present results
Some exoplanets
HD 188753 Ab – This was the first exoplanet discovered in a
multiple stellar system (three stars).
HD 209458 b e HD 189733b – The first exoplanets whose spectrum
was analyzed by direct observation.
Gliese 581 c – This planet seems likely to harbour liquid water on its
surface – a basic requirement for life. No strong clues supporting the
existence of water have been found – yet, the planet is at a suitable
distance from the parent star to have the right temperature interval
allowing for liquid water. According to the estimates, the planet
should be about 50% larger than Earth, and five times more massive.
The search for extrasolar planets
Some exoplanets
Our Solar System, compared with 55 Cancri’s
planetary system
In this image the inner Solar System is superimposed
to the orbit of some exoplanets: HD 179949 b,
HD 164427 b, e Reticuli A b, and m Arae b
The results
The mass distribution
Number of planets vs. mass
Number of planets (95)
Number of planets (164)
Number of planets vs. mass
Planet mass (MJ)
Planet mass (MJ)
Left: the mass distribution for smaller exoplanets (M <1 MJ). Right: the mass distribution
for larger exoplanets (M >1 MJ). MJ = 1 Jupiter mass = 318 Earth masses
The results
The distance distribution
Number of planets vs. semi-major axis
Number of planets (8)
Number of planets (247)
Number of planets vs. semi-major axis
Semi-major axis
Semi-major axis
Left: exoplanets that are closer to their star than Jupiter is to the Sun. Right: exoplanets
that are farther from their star than Jupiter is to the Sun. For comparison, Neptune’s
distance from the Sun is 30 AU
The results
Mass of planet (MJ)
Mass vs. semi-major axis
Jupiter
Semi-major axis (AU)
The results
Mass of planet (MJ)
Mass vs. semi-major axis
Semi-major axis (UA)
The results
Considerations
The results obtained so far are obviously incomplete, all methods
used being strongly biased towards detection of large-size planets.
The discovery of so many ‘hot Jupiters’ prompted a critical
discussion and several attempts at reworking the theory of formation
of planetary systems – which, in turn, relies upon classical solar
nebula models.
Our observations
The beginnings
On July 2007, following a suggestion by Rodolfo Calanca (vice-editor of
COELUM Astronomia Magazine as well as Planetary Reseach Team’s
co-ordinator), our group joined the national project “Search the Sky!”.
Focus of the project was the detection and study of extrasolar planets.
Such a task was to be performed by powerful telescopes coupled to highquality charged couple devices (CCDs) - by now common enough
among Italian amateur astronomers.
Our observations
The beginnings
On 26 July, 2007, with our
instruments (a home-built
f/4.5 – 410mm reflector
telescope, plus a SBIG ST7–
ME CCD) we began
observing transits of
exoplanets.
The task of taking images
and the data reduction were
performed by making use of
available on-line software.
Setting up the f/4.5-410mm telescope.
From left to right: Angelo Angeletti, Francesco
Barabucci, Fabiano Barabucci and Gianclaudio
Ciampechini.
Our observations
26 July, 2006 – TrES 2
-0,47
0,44
0,46
0,48
0,5
0,52
Dm (= 11 – m)
-0,48
-0,49
-0,50
-0,51
Dt (= t – 2454308) JD
TrES = Trans-atlantic Exoplanet Survey
0,54
0,56
Our observations
11,070
11,075
11,080
11,085
11,090
11,095
11,100
22.58
22.48
22.38
22.28
22.18
22.08
21.58
21.48
21.38
21.28
21.18
21.08
20.58
20.48
31 July, 2006 – TrES 2 again
Our observations
5 and 12 August, 2006: HD209458
On 5 and 12 August, 2006 we have tried to image the transit of
HD209458 - the first planet whose transit was imaged using amateur
instruments.
Alas, we failed.
First failure was caused by bad weather. We are still trying to figure
out what went wrong on the second attempt!
11,845
11,850
11,855
11,860
11,865
11,870
11,875
11,880
11,885
11,890
1.10
1.00
0.50
0.40
0.30
0.20
0.10
0.00
23.50
23.40
23.30
23.20
23.10
23.00
22.50
22.40
22.30
22.20
22.10
22.00
21.50
21.40
21.30
21.20
21.10
21.00
20.50
Our observations
17 August, 2006 – TrES 4
Our observations
1 September, 2006 – TrES 2 again
11,040
11,045
11,050
magnitude
11,055
11,060
11,065
11,070
11,075
11,080
11,085
2.05
1.55
1.45
1.35
1.25
1.15
1.05
0.55
0.45
0.35
0.25
0.15
0.05
23.55
23.45
23.35
23.25
23.15
23.05
22.55
22.45
22.35
22.25
tim e (UT)
Our observations
14 September, 2006 – WASP 1
11,565
11,570
11,575
magnitude
11,580
11,585
11,590
11,595
11,600
11,605
WASP = Wide Angle Search for Planets
3.30
3.15
3.00
2.45
2.30
2.15
2.00
1.45
1.30
1.15
1.00
0.45
0.30
0.15
0.00
23.45
23.30
23.15
23.00
22.45
22.30
tim e (UT)
Our observations
15 September, 2007 – TrES 1
11,330
11,335
11,340
magnitude
11,345
11,350
11,355
11,360
11,365
11,370
11,375
0.30
0.15
0.00
23.45
23.30
23.15
23.00
22.45
22.30
22.15
22.00
21.45
21.30
21.15
21.00
tim e (UT)
Our observations
The present
Our trial-and-error approach has eventually provided a suitable
step-by-step observation sequence, resulting in high-precision
imaging of transiting exoplanets.
The next step in our schedule: devising and implementing a
new observational method enabling the discovery of a new
exoplanet by the transit method.
(It may be worthwhile to remark that no amateur astronomer
has discovered a new extrasolar planet yet)
Future work
All future work is devoted to a
single aim:
discovery of Earth-like planets
lying within the habitable zone
of their planetary system.
The image to the right displays theoretical limits of the next
generation instruments – either Earth- or space-based – in
detecting exoplanets until the year 2015 (lines in colour)
(P.R. Lawson, S.C. Unwin e C.A. Beichman, 2004)
Future work
The habitable zone
The habitable zone of a planetary system is the region where a rocky
planet might harbour liquid water under stable conditions
Future work
Astrometry
The ESO (European Southern Observatory) is planning a groundbased search for giant planets orbiting some hundred nearby stars;
the project is scheduled to start by 2008. It will make use of the
PRIMA device, which will be installed upon the great 120-m VLTI
(Very Large Telescope Interferometer), which is located in the
Chilean Andes.
Future work
Astrometry
Two space-based projects are completing the preliminary phase:
- SIM (Space Interferometry Mission), by NASA, a 20-m
interferometer placed upon a beam, is composed of two 40-cm
telescopes. Its launch is scheduled for the year 2009.
SIM will search for exoplanets around 1500 stars (among the closest
to Sun); the device is sensitive enough to detect exoplanets of some
terrestrial masses at a distance of less than 15 light years dal Sole.
- GAIA, by ESA, a device measuring the reciprocal positions of the
stars (brighter than the magnitude 20) and their changes with time.
GAIA will be able to detect any change in the position of 1,5 billion
stars. Its accuracy is high enough to detect Jupiter-sized exoplanets
around 20000 stars. Launch is on schedule for the year 2012.
Future work
Transits
Hundred of small- and middle-sized telescopes (up to one metre) are now
active throughout Europe, working hard to detect ‘hot Jupiters’ by taking
advantage of transits.
As for space-based research, the French space agency (CNES) - in
partnership with other European countries - launched this year CoRoT, a
30-cm telescope whose task (among others) is the search for planetary
transits over 60000 stars.
CoRoT is sensitive enough to successfully detect exoplanets as massive
as twice the Earth.
Future work
Transits
An artist’s impression of the CoRoT Project
Future work
Direct detection
Direct detection is by far the most promising approach in the future.
It allows a detailed study of chemical and physical properties of
exoplanets: the atmosphere (density and composition),
surface properties (colour, oceans/continents morphology)
rotation (length of the ‘day’), satellites and rings.
Several projects are in schedule, ground- as well as space-based.
Most activities are focussed on this area – by now a
rapidly-expanding field.
Future work
Ground-based observation
By 2008, ESO is scheduled to
activate an imaging device called
Planet Finder, which is supposed
to operate on one of the 8-m
mirrors of the VLT (Very Large
Telescope), based in Chile.
Very Large Telescope
The Keck 10-m telescope has a
similar project on schedule.
Keck Telescope
Future work
Ground-based observation
The American LBT (Large Binocular
Telescope), based in Chile, is composed of a
pair of twin 8,2-m telescopes. One of them is
equipped with a special camera, especially
designed for the search of exoplanets.
Both United States and Europe have in store
long-term projects, involving even bigger
telescopes. Their diameter will be somewhere
between 30 and 100 metres.
Such telescopes (not scheduled before 2020!)
will be equipped with last-generation
imaging devices, aimed to the discovery of
Earth-sized planets.
Large Binocular Telescope
Owl Telescope
Future work
Space-based observation
For the time being - and
not counting old Hubble
Space Telescope (HST) –
only one space-based
telescope is scheduled for
launch in 2011: the James
Webb Space Telescope
(JWST).
JWST, a 7-m telescope, is
optimized for infrared
observation. It should be
able to detect any
exoplanet orbiting the stars
closest to Sun.
Future work
Middle-term projects in space-based observation
The NASA-conceived TPF-C
telescope (Terrestrial Planet
Finder Coronagraph) is planned
for detection of Earth-size planets
by making use of reflected
starlight (right)
Future work
Middle-term projects in space-based observation
The most popular idea is building an
interferometer composed of several
3-m telescopes (the number can vary
anywhere from threee to six), placed
some tens or hundreds metres apart.
One of the four
or five Darwin
telescopes
Two projects are scheduled: Darwin
(ESO) and TPF-I (NASA).
Both projects are supposed to search
for Earth-sized planets by detecting
their thermal emission.
TPF-I – Terrestrial Planet Finder
Interferometer
Future work
Conclusions
There’s a lot of excitement about Darwin, TPF-C, and TPF-I
projects. Their accomplishment will make possible a direct search
for traces of biological activity in exoplanets’ spectra. Detecting the
signature of life somewhere outside Earth would at last fulfill an
age-old hope, which is deeply rooted in human mind:
We are not alone!