→ Widely unknown Exoplanet Astronomer

Credit: NASA

Exomoons are moons expected to orbit exoplanets. Although no exomoon discovery has been published to date, there is no doubt that we will find them.

In a recent paper by Simon et. al titeled: Signals of exomoons in averaged light curves of exoplanets they set out to suggest a new method for discovering these exomoons, the so called “Scatter Peak” method. The idea is to study the local scatter in a number folded lightcurves (ideally a 100 or more). It is thought to that this method will allow the discovery of moons around planets with a period of 10-20 days assuming the observations are done during 3 to 5 year long observing campaigns using space observatories.

I find the Scatter Peak method for detecting exomoons very promising provided the three conditions imposed by the authors of the paper are met:

1. The stacking of the individual lightcurves has to be extremely accurate  so that the transit times coincide.
2. The transit observation has to have a continuum (flat part of the lightcurve) which is at least as long as the transit duration itself.
3. The trend filtering must be done so that small deviations immediately before and after the transit of the exoplanet remain unaffected.

A great resource to find out more about exomoons is the recently submitted PhD thesis of David Kipping titled:

“The Transits of Extrasolar Planets with Moons”

An observed dip in the transit lightcurve does not automatically infer the detection of an exoplanet. There are three common types of false exoplanet candidates which astronomers encouver all the time.

This NASA/ESA Hubble Telescope near-infrared image of newborn binary stars (image center) reveals a long thin nebula pointing toward a faint companion object. Credit: Susan Terebey (Extrasolar Research Corp.), and NASA/ESA

Grazing binary systems

Usually binary stars cause big dips in the lightcurve. In some cases though the closer of the two binary stars grazes the limb of the other star (as seen from earth) and a small dip, similar to that of an exoplanet lightcruve, is observed. Some differences can be spotted though. For one, the lightcurve from grazing binary stars will have a distinct V shape whilst an exoplanet crossing the surface of the star will have more of a U shape with a flat central bottom caused my the whole exoplanet occulting the host star. Also, if the grazing eclipsing binary system has a circular orbit, two conjuctions will happen, each star alternating with one star infront of the other. Thus a full orbital lightcurve will contain two eclipses of different lightcurve depths. There is an excpetion however, and that is if the eclipsing binaries are main sequence stars of similar mass (i.e same radius and luminosity). To determine if it is in fact an exoplanet or not, the lightcurve shape has to be examined or radial velocity followup observations will have to be done.

Blended eclipsing binary systems

This is more of a problem with exoplanet surveys using small (< 1 m) telescopes where stars are more likely to not be spacially resolved. A shallow dip in the lightcurve resembling an exoplanet transit can occur when a deeply eclipsing binary star happens to lign up with another isolated star along the same line of sight. The isolated star acts to diminsh the effects of a big flux change which would have otherwise occured had the observations of the deep eclipsing binary not been along the line of site of the isolated star. For larger telescopes this is not so much of a problem as the stars are more likely to be spacially resolved with their light hitting different pixels on the detector.

Credit: NASA/Wendy Stenzel.

Brown dwarf and white dwarf stars

The transiting lightcurve depends only on the size of the transiting object. Brown dwarfs and white dwarfs have similar sizes to the giant gas planets and thus can be mistaken as planets despite the fact that the giant gas planets are much less massive. To get around this a radial velocity curve of the host star will have to be obtained in order to estimate the minimum mass. The potential planet is given said to be a exoplanet candidate until such an observation is done.

The beauty of exoplanet transits is that they can be observed with amateur equipment and doesn’t require that you have access to a professional telescope located at a prime observing site. To be able to succeed at observing a transit you do need to know what you are doing. Here I will very briefly mention things to keep in mind.

Equipment needed

First off, good data analysis skills are very important and should not be underestimated. If you don’t know what you are doing and why, it won’t really matter how good your equipment is. To be able to observe exoplanet transits you will need a telescope which is able to track the sky accurately. An auto guiding system is essential. The greater the aperture of the telescope the greater signal to noise you will get and the easier you will detect the exoplanet. An 8 inch telescope will be able to detect exoplanets orbiting a star of magnitude 10 or less. With a 12 inch most transiting exoplanets will be available for observing.

Detector needed

Any 16 bit CCD camera should do the job. Dark current for instance is not so important as the exposures taken are about 30 seconds long.

Targets

The transiting exoplanets which you will be able to observe will depend on the size of the telescope you have. I would recommend to start of with the brightest objects such as HD 209458 or HD 189733.

Software

Once you have the data there are a number of ways which you can reduce the images. Examples are IRAF and MaxIm

Resources:

I highly recommend a great Ebook by Bruce L. Gary: Exoplanet observing for amateurs

A presentation by Michael Theusner: Exoplanet transit observations with amateur equipment