Transcript Exoplanets for Amateur Astronomers

Exoplanets for Amateur Astronomers
• What is an Exoplanet?
• How are they being found?
• What is the role of Amateur Astronomy in Exoplanet research?
Taken with Subaru telescope in near infrared
What is an Exoplanet
• An extrasolar planet, or exoplanet, is a planet outside the
Solar System.
• Astronomers have suspected and searched for
exoplanets since mid 19th century, but first scientifically
confirmed discovery was found in 1995 (51 Pegasi)
• As of January 2010, 429 have been discovered
• Majority of ones found so far are massive, “Hot Jupiter”
types (mainly due to ease of discovery) but recent
discoveries like Gliese 581 d are approaching what we
would call “terrestrial”.
How do you find an Exoplanet?
Four main methods:
• Astrometry
• Radial Velocity
• (Photometric) Transit
• Gravitational Microlensing
Gravity…
(warning – Science Content!)
• Gravity is the means by which objects with mass attract
one another.
• F = G(m1*m2)/r2
• Bigger mass = more force, smaller distance apart = more
force
Finding an Exoplanet - Astrometry
• Astrometry consists of precisely measuring a star's
position in the sky and observing the ways in which that
position changes over time. If the star has a planet, then
the gravitational influence of the planet will cause the
star itself to move in a tiny circular or elliptical orbit
around the common center of mass.
Doppler Effect…
(warning – Science Content!)
• The Doppler Effect (or shift) is the change in frequency of a
wave for an observer moving relative to the source of the
wave. It is commonly heard when a vehicle sounding a siren
or horn approaches, passes, and recedes from an observer.
The received frequency is higher (compared to the emitted
frequency) during the approach, it is identical at the instant of
passing by, and it is lower during the recession.
Finding an Exoplanet – Radial Velocity
• As the star moves in its small orbit around the system's center of
mass, its velocity (from Earth prospective) also changes.
Variations in the star's radial velocity - that is, the speed with which
it moves towards or away from Earth — can be deduced from
displacements in the star's spectral lines due to the Doppler effect.
Extremely small radial-velocity variations can be detected, down to
roughly 1-2 m/s. (On January 7, 2010, the second least massive
radial velocity planet, HD 156668 b, was discovered by HARPS*
with the lowest semi-amplitude ever measured, at 1.89 m/s)
• This has been by far the most productive method of discovering
exoplanets.
*High Accuracy Radial-velocity Planet Searcher
Radial Velocity – Cont.
• Radial velocity works well if rotation is in an edge on plane with the Earth,
but not if rotation is in a face on plane. Assuming perfect edge on plane,
mass of planet can be determined. Also, multiple (large) planets in an
exosolar system can cause conflicting measurements.
• First studies assumed Jupiter like solar system – you needed years of
observations to determine if a planet existed (Jupiter orbital period is 4332
days). However, first planet found (51 Pegasi b) had a orbital period of 4
days!!!
• Current equipment can measure deviations down to 1-2 m/s. For point of
reference, Jupiter causes deviation of 12 m/s and the Earth .09 m/s. This
makes Radial Velocity not ideal for finding Earth sized objects.
Finding an Exoplanet – Transit
• This method uses the fact that when a smaller and less bright object
passes in front of a bright object, such as a star, the star appears to
fade in luminosity. Even if the reduction is very small (typically
between 0.01% and 1%) astronomers can detect it. The object
passing by could be a star, brown dwarf or a planet. The event could
be regarded as an eclipse or an "occultation".
• The photometric transit method has an disadvantage in that the star
which is being studied needs to be edge-on relative to the line of
sight of the observer. Less than 1% of the stars (F-, G and K class
dwarfs are the most promising candidate stellar types) would have
this kind of desired orbit.
• Transit exoplanet discoveries also need to be confirmed as they
could be caused by observing conditions, starspots, etc.
Finding an Exoplanet – Transit
continued
Quick Quiz! What do you think caused the
following exoplanet transit observation?
Finding an Exoplanet – Transit
Continued
• What happens if you combine knowledge from
Radial Velocity and Transit?
• Radial Velocity gives a minimum mass estimate
(based on gravitational effect) and orbital period
• Transit gives % light blocked (which tells how big
the object is) and length of transit.
• Combine the two… combine orbital period and
length of transit and determine angle of rotation
(how close to edge on) – this can be used to
adjust the mass estimate. Combine mass
estimate with size and you get density, which
helps to determine composition.
Finding an Exoplanet – Transit
Continued…
• But wait – there’s more!
• The transit method also makes it possible to study the atmosphere of the
transiting planet. When the planet transits the star, light from the star
passes through the upper atmosphere of the planet. By studying the
high-resolution stellar spectrum carefully, one can detect elements
present in the planet's atmosphere. A planetary atmosphere (and planet
for that matter) could also be detected by measuring the polarisation of
the starlight as it passed through or is reflected off the planet's
atmosphere.
• Also, the secondary eclipse (when the planet is blocked by its star)
allows direct measurement of the planet's radiation. If the star's
photometric intensity during the secondary eclipse is subtracted from its
intensity before or after, only the signal caused by the planet remains. It
is then possible to measure the planet's temperature and even to detect
possible signs of cloud formations on it.
Finding an Exoplanet – Transit
Continued…
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Kepler and COROT
COROT (COnvection ROtation and planetary Transits) is a space mission led by
the French Space Agency (CNES) in conjunction with the European Space Agency
(ESA) and other international partners.
COROT has 4 CCD detectors. These detectors are arranged in a square pattern
with two each dedicated to the planetary detection and asteroseismology. The field
of view for planetary detection is 3.5°. It will observe perpendicular to its orbital
plane, meaning there will be no Earth occultations, allowing 150 days of continuous
observation. During the northern summer it will observe in an area around Serpens
Cauda and during the winter it will observe in Monoceros. To date, COROT has
detected 7 new exoplanets.
The Kepler Mission uses a NASA space observatory designed to discover Earthlike planets orbiting other stars. The spacecraft was launched on March 7, 2009
and is named in honor of German astronomer Johannes Kepler. With a planned
mission lifetime of at least 3.5 years, Kepler uses a photometer developed by
NASA to continuously monitor the brightness of over 145,000 main sequence stars
in a fixed field of view. The field of view is roughly 10x10 degrees and is made up
of 42 2.25 megapixel CCD’s.
Finding an Exoplanet –
Gravitational Microlensing
• Microlensing is a phenomenon that occurs when an object with
enough mass (examples: planet, brown dwarf or a low mass star)
passes between us and a background star. If, for example, a planet
and a star would happen to pass in front of such a background star,
the background star's luminosity would appear to increase (light is
bent by the planet-starsystem's gravity). A passage that lasts for
long (days, weeks) would indicate that the planet is orbiting at a
greater distance than it would have, if the passage only lasted for
hours.
• This is a very promising and new method, though the chance is slim
that a planet-starsystem would pass between us and a background
star. For this reason, it is more efficient to study a background with
many stars, for example a view towards the galactic centre would
provide a significant amount of stars.
• Using microlensing, astronomers have discovered a gas giant of
three Jupiter masses orbiting a star at an amazing distance of
15,000 light years
Finding an Exoplanet – The role of the
Amateur
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(July 7,2005) “…on June 30th, California amateur astronomer Ron Bissinger detected a
partial transit of planet HD 149026b. He also detected partial transits during the next two
opportunities, allowing him to produce a composite light curve of an entire event. This new
find is now the third transiting exoplanet detected by amateurs.”
Although the spectographs needed for Doppler Spectroscopy are outside the realm of an
amateur astronomer (most discoveries to date have been made with the HARPS
spectograph at the 3.6-meter ESO telescope in Chile), the CCD’s to do transit studies are
well within the reach of amateur astronomers. Biggest issue with measuring transits is
signal to noise ratio, which means that for earth bound observations, transparency and
stillness of sky are more important that size of scope. Confirming exoplanets by transit
method also requires multiple measurements to rule out false positives, so it is time
consuming (bigger the orbital period, the longer between measurements, etc.)
Kepler and COROT measurements will be better than amateur ability, but they are focused
on very small parts of a very big sky, leaving much to be measured by other means.
“Professional’ scopes are very limited and in high demand.
Transitsearch.org has collected a number of suspected exoplanets that need confirmation
and are available for the amateur community to contribute to “real science”. Many of these
suspects have a <1% chance of being a true planet (too low for academic/professional
scopes to be used) but having a 1% chance of confirming a new exoplanet are great odds
for an amateur.
Finding an Exoplanet – The role of the
Amateur
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What do I need to “see” an Exoplanet?
Telescope (12” or larger, tracking)
CCD (the more megapixels the better)
Software to generate curve
HD 149026b
Relatively dark skies
A target (http://transitsearch.org)
A guide (http://brucegary.net/book_EOA/x.htm)
Practice and Patience
Finding an Exoplanet – The role of the
Amateur – What’s Next?
• The continued search for “Earth-sized” exoplanets (such as
COROT-Exo-7b, 1.7x bigger radius than Earth, but with an
orbital period of 0.854 days, so probably not too habitable! )
• The search for life signs on exoplanets will expand, including
study of atmospheric spectra (lower red wavelengths,
increased Oxygen and Methane are all possible signs of
life), in addition to study of size and composition of planet,
distance from star, ambient temperature and so forth.
• The search for Exomoons will begin and should involve
amateurs as it will most likely be done by studying minute
variations in the transit time for known transiting planets.
Questions???