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

• 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”.

What is an Exoplanet

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 – TransitContinued

• 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 – TransitContinued…

• 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 – TransitContinued…

• 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 Earth-like 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

• (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

• What do I need to “see” an Exoplanet?• Telescope (12” or larger, tracking)• CCD (the more megapixels the better)• Software to generate curve• Relatively dark skies• A target (http://transitsearch.org)• A guide (http://brucegary.net/book_EOA/x.htm)

• Practice and Patience

HD 149026b

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???