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General Astronomy
Formation of Solar Systems
Observational Evidence
And Some Surprises
“Are there somewhere, other worlds like ours?”
-- Epicurus, 300BC
Copyright 2003 Kimmo Isokoski
Other Worlds
After 2,300 years, we can finally answer
the question.
“Yes, Epicurus. There are other worlds
orbiting about distant suns.”
By Spring 2015, we had discovered over
1915 planets / 1210 planetary systems /
481 multiple planet systems
Beginning the Search
"We shall be less apt to admire what this World calls great...when
we know that there are a multitude of such Earths inhabited
and adorn'd as well as our own."
"...so many Suns, so many Earths ...And how must our Wonder and
Admiration be increased when we consider the prodigious
Distance and Multitude of the Stars."
Christiaan Huygens, ‘The Celestial Worlds Discover'd: Conjectures
Concerning the Inhabitants, Plants, and Productions of
the Worlds in the Planets’, 1690
Of course, in 1690, there was little chance of actually
observing planets about stars other than our sun. We
have had to wait 300 years before it became possible
Beginning the Search
In 1916, E.E. Barnard published a report of a star with the
extremely large proper motion of 10.3 arcseconds per year.
It is a small, red star named (appropriately) ‘Barnard’s Star’
It’s motion is large because it is only about 5.9 lightyears
distant.
In fact, it is moving toward us (or we’re moving toward it)
and it will only be 3.8 lightyears away by 11,800 AD
Because of the motion, and distance, it is a good candidate
to look for a planetary system.
Barnard’s Star
• Starting in the late 1930’s, Peter van de Kamp
began a lifelong project devoted to proving
Barnard’s Star harbored planets.
• Working at Sproul Observatory (Bryn Mawr
College, Philadelphia), he began taking
accurate photographs of Barnard's Star
• Using 2000 photos (1937 – 1962), van de Kamp
looked for a telltale wobble in the proper
motion of the star against be background of
the more distant stars.
Methods
Before discussing van de Kamp's
results, let's look a the ways an
extra-solar planet might be detected:
•
•
•
•
Astrometry
Direct Observation
Radial Velocity
Photometry
Astrometry
Astrometry is the precise measurement of
the positions of astronomical objects.
A star and planet act gravitationally on each other. The
planet is pulled about the star and vice versa. In reality,
they both orbit a common center of mass (generally
nearer to the star).
This causes a wobble in its path as seen against the
background of distant stars
Looking for the Wobble
Proper Motion
No Planets
1940
One or more planets
1960
1980
2000
Direct Observation
• Based on observing the reflected light of
the parent star
• Only extremely large planets may be
detected using this method
• The overwhelming brightness of the parent
star makes the planet very hard to detect
• Another method for direct observation
looks for the IR signature of a dust disk
surrounding the star and can, in fact, even
locate gaps in the disk indicating the
presence of planets.
Dust about 55 Cancri
Hubble’s view of
protoplanetary disks
Radial Velocity
• As in astrometry, this technique looks
for a wobble in the star’s motion.
Instead of looking for small side-toside movement (I.E. in proper motion)
this looks for wobbles in the radial
direction using doppler shift.
Looking for the Wobble
Radial Velocity
Photometry
• This method measure very precise values
of a star’s brightness. When a planet
‘transits’ the star, it’s brightness dims by
a very small – but measurable - amount.
• Based on the shape of the resulting
lightcurve, a planet may be inferred
Planetary Transit
Kepler
KEPLER 2009
Detection
ofof
Detection
Exoplanets:
Exoplanets:
Planetary
Planetary
Transit
Method
Transit
Method
Photometry of HD209458
Where to Look?
Nearby stars are the best because:
Planets are brighter.
The angular separation between planets and the star is larger
Stars are brighter
Doppler shifts easier to measure
Motions on the sky easier to measure
Stars with distance less than 10 parsecs from the Sun
Spectral
Class
Mass
M
NTotal
NSingle
A
1.8 - 3.4
2
0
F
1.1 - 1.8
11
5
G
0.8 - 1.1
26
13
K
0.5 - 0.8
42
18
M
< 0.5
210
63
Back to Barnard's Star
Controversy and more controversy.
Over the years, van de Kamp refined his calculations and published changes
to ‘his planets.’
Originally a single very massive planet in an eccentric orbit; they became 2
planets, one slightly bigger than Jupiter and the other slightly smaller in
nearly circular orbits.
By 1985, when van de Kamp published a final paper on his discovery, other
astronomers, using different telescopes, could not find the distinctive
wobble. One went so far as to suggest that the wobble was in the
Sproul Telescope rather than in the star. Others claimed they detected
a wobble, but not the same as van de Kamp (his rebuttal reflected the
fact that he had spent over 40 years looking a thousands of
photographic plates and therefore knew best).
At this point in time, the ‘jury is still out’ and we really don’t know if
Barnard’s Star was the first discovery of extra-solar planets.
Types of planets
•
•
•
•
Classic Jovian
Epistellar Jovian
Terrestrial
Pulsar Planets
A sampling
of
extrasolar
systems
New Earth-like Planet!
The star is a red dwarf with spectral type M3V, located 20.3 light-years away from Earth.
?
The First Images of Exoplanets
One planet orbiting
Fomalhaut
Star
location
• New images show planets
orbiting bright young
nearby stars
• Although more than 350
planets are known to
orbit other stars, none
could be imaged until now
Neptune-sized
orbit
Hubble Space Telescope visible image of the star Fomalhaut
(whose light was blocked), with a dust belt similar to the
Kuiper belt. Inset: Images taken ~2 years apart show a planet
moving around the star.
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009
One of a few images of an
exoplanet around a star
Glowing Young Planets
• This star has three orbiting planets the first imaged planetary system!
• Planets are much fainter than their
parent star, so are difficult to image
• Why are these pictures possible?
• Advanced observing techniques
were used to block the star’s
light
• Observations were repeated over
years, confirming planetary
motion
• The planets are young and hot,
and therefore glow more brightly
than by reflected starlight alone
Three planets
orbiting HR8799
Keck Observatory infrared image of star
HR8799 and three orbiting planets with orbital
directions indicated by arrows. The light from
the star was subtracted, but a lot of ‘noise’
remains.
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009
The Big Picture
• Previous exoplanet detections
were indirect (used stellar
motion and transit methods)
• These are the first pictures
of planets around other stars
• Ever-improving images from
Earth and space should allow
us to detect smaller (more
Earth-like) planets
• Images and spectra at
different wavelengths will
allow us to measure the
composition of exoplanet
atmospheres, and determine
whether they are habitable.
Artist’s conception of a planetary system orbiting another
star.
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009
First Rocky Exoplanet Detected
• Most known exoplanets are large
and have low densities - similar
to jovian planets in our solar
system
• A space telescope recently
discovered a planet with radius
only 70% larger than Earth’s
• Groundbased observations show
the planet’s mass is less than 5
times Earth’s
• Together, the observations
reveal that the planet’s density
is similar to Earth’s - the first
confirmation of a “rocky”
exoplanet
Artist’s conception of the view of the rocky planet’s
parent star (Corot-7) from above the surface of the
planet (Corot-7b). Image from ESO / L. Calcada.
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009
How Can We Find a Planet’s Density?
• Density = Mass / Volume
• The planet’s mass was determined
using the radial velocity method:
Changes in a planet with mass ~5 times Earth’s. the
measured wavelengths of star light are caused by
• Volume = 4/3  R3
• The planet’s size was determined
using the transit method:
The amount of light measured
from a star decreases when a
planet passes in front. The
amount of decrease indicates the
planet’s size.
Amount of Light
The planet gravitationally ‘tugs’ on
the star, shifting the wavelength
of light the star emits back and
forth. The amount of shift
indicates the planet’s mass.
-4
0.04%
-2
0
Hours
2
4
Periodic decreases in light from the star are caused
by a planet with diameter 1.7 times Earth’s passing in
front.
The Big Picture
• After discovering hundreds of
exoplanets resembling our jovian
planets, astronomers have found
the most Earth-like planet to date
• Although planet Corot-7b’s density
is close to Earth’s, differences
abound: it orbits its star in ~20
hours (faster than any known
exoplanet) - so close that its rocky
surface may be molten
• With the existence of Earth-like
planets now demonstrated,
astronomers have reason to hope
that the Kepler mission will
discover more
Detection of more rocky exoplanets (‘Super-Earths’)
like those in this artist’s depiction should come
rapidly, thanks to dedicated space telescopes and
improving ground-based detection capabilities.
Image from D. Aguilar, Harvard Smithsonian CfA.
Kepler 186f
A newly discovered (17 APR 14) planet nicknamed
"Earth's cousin" has been found 492 light-years
from Earth.
The planet, called Kepler-186f is the first Earthsize planet found in the habitable zone of its star.
Ten percent larger than Earth, Kepler-186f is the
closest planet to Earth in size ever found in the
habitable zone of its star.
Artist’s Concept
The newly discovered exoplanet
orbits about 32.5 million miles
(52.4 million kilometers) from
its sun. It takes Kepler-186f
about 130 days to orbit its red
dwarf star.
The Kepler-186 star is about
half the mass of the sun, and
the newly discovered planet is
far enough away from its star
that powerful flares may not
greatly affect the planet.
To be considered…
• Assuming that the planets are ‘real’ how
does this change our models of the
formation of solar systems?
• First, it helps to show that solar systems
are not freak occurrences, but common,
natural phenomenon
• We have to reconsider, how gas giants can
form near a star (epistellar Jupiter's)
rather than far out where it is cool.
Press Releases
•
For More Information…
Space.com - 09/16/09 - “First Rocky World Confirmed Around Another Star”
http://www.space.com/scienceastronomy/090916-rocky-exoplanet.html
•
Eurpoean Southern Observatory - 09/16/09 - ‘First Solid Evidence for a Rocky Exoplanet’
http://www.eso.org/public/outreach/press-rel/pr-2009/pr-33-09.html
Images
•
Artist depiction of Corot-7 system courtesy European Southern Observatory / L. Calcada
http://www.eso.org/public/outreach/press-rel/pr-2009/pr-33-09.html
•
•
•
Detection method cartoons - 2006 Pearson Education Inc., publishing as Addison Wesley
Transit and radial velocity data plots adapted from source articles below
Artist depiction of Super-Earth courtesy David Aguilar, Harvard Smithsonian CfA
http://www.cfa.harvard.edu/news/2008/pr200802_images.html
Source Articles
•
(on-campus login may be required to access journals)
Léger et al., ‘Transiting exoplanets from the CoRoT space mission VIII. CoRoT-7b: the first
Super-Earth with measured radius’, Astronomy and Astrophysics, in press, 2009.
http://www.aanda.org/articles/aa/pdf/forth/aa11933-09.pdf
•
Queloz et al., ‘The CoRoT-7 planetary system: two orbiting super-Earths’, Astronomy and
Astrophysics, in press, 2009.
http://www.aanda.org/index.php?option=article&access=doi&doi=10.1051/0004-6361/200913096
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 3 December, 2009
For more information…
Press Release / News Story
•
New York Times - 11/13/2008 - “Now in Sight: Far-Off Planets”
http://www.nytimes.com/2008/11/14/science/space/14planet.html
Images
•
Hubble image of Fomalhaut B
Taken from the source article by Kalas et al.
•
Keck image of HR8799
Taken from the source article by Marois et al.
•
Artist concept of another planetary system from Gemini Observatory
http://tinyurl.com/geminiplanetfamily
Source Articles
•
(on-campus login may be required to access journals)
Kalas et al., ‘Optical Images of an Exosolar Planet 25 Light-Years from Earth’,
Science, 322(5906), p. 1345 DOI: 10.1126/science.1166609.
http://www.sciencemag.org/cgi/content/abstract/322/5906/1345
•
Marois et al., ‘Direct Imaging of Multiple Planets Orbiting the Star HR 8799’,
Science, 322(5906), p. 1348 DOI: 10.1126/science.1166585, 2008.
http://www.sciencemag.org/cgi/content/abstract/322/5906/1348
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009
Links
http://exoplanets.org/
http://exoplanet.eu/index.php
http://www.public.asu.edu/~sciref/exoplnt.htm
http://www.astro.uiuc.edu/~kaler/sow/pp.html
http://www.msnbc.msn.com/id/11022898