Exoplanet dip in transit light curve when the planet passes behind the star

Exoplanet dip in transit light curve when the planet passes behind the star

In the animation below, I don't understand why the brightness slightly decreases when the planet is behind the star.

Where does this effect come from?

Just before the planet goes behind the star, we see the light directly from the star as well as the light reflected from the planet's surface. When the planet is behind the star, we no longer see the reflected light.

Note that the top of the line also curves to reflect how much of the planet's surface is illuminated from the star. The most illumination is when it is almost "full" just before it passes behind the star. The least illumination is when it is "new" just before it passes in front of the star.

Edit (to address Fraxinus's comment below): Here is an example from Zhang et. al: Phase Curves of WASP-33b and HD 149026b and a New Correlation between Phase Curve Offset and Irradiation Temperature.

The above graphic shows processed data from the Spitzer space telescope. From the paper:

Observations were timed to begin before a secondary eclipse and end after the following secondary eclipse,

These light curves help aid in estimates of planetary albedo, day/night temperature, and atmospheric composition.

Planet-Hunting Spacecraft Shows Its Stuff by Detecting a Known Exoplanet

NASA's Kepler spacecraft, which may soon help scientists put our planet in its galactic context by showing how common Earth-like worlds are throughout the Milky Way, is off to a good start.

The space telescope, which was launched in March and began its science mission in May, will spend more than three years observing a patch of 100,000 stars near the northern constellations Cygnus and Lyra. If those stars have planetary systems aligned with Kepler's line of sight, the spacecraft's photometer should be able to detect the periodic dimming caused by the planets as they transit, or pass in front of, their stars.

Hundreds of exoplanets&mdashplanets beyond our solar system&mdashhave already been detected from the ground and from other spacecraft via transit searches and other methods. But the current exoplanet catalogue primarily reflects the low-hanging fruit&mdashextremely large planets in tight orbits, whose visible or gravitational effects on their stars are more pronounced. Many of these planets are known as hot Jupiters.

Kepler's mission is to seek out smaller worlds more like our own, ideally in comfortable, life-enabling orbits in their respective stars' so-called habitable zone.

A paper in this week's Science, using 10 days of early data gathered by Kepler, demonstrates the spacecraft's ability to spot large planets and provides encouragement that Earth-size bodies are within its reach.

In the study, the Kepler team shows that the data clearly reveal the dimming caused by the periodic transit of HAT-P-7 b, an exoplanet nearly twice the mass of Jupiter that orbits the star HAT-P-7, about 1,000 light-years away. The exoplanet, discovered last year by ground-based observatories, orbits so close to its star that it completes a loop in just 2.2 days&mdashmaking it a very hot Jupiter.

"It is so hot, in fact, that it glows, like the heating element in your oven or toaster," says Kepler deputy principal investigator David Koch of NASA Ames Research Center at Moffett Field, Calif. In addition to the dip in starlight as HAT-P-7 b passed in front of its star, Kepler was able to detect the exoplanet's glow, which appears in the data as increased emission from the star when the planet is visible alongside of it.

Crucially, Kepler also detected a slight dip in luminosity, much less dramatic than the dimming associated with the planet passing in front of the star, when HAT-P-7 b passed behind its star&mdashthe spacecraft was seeing only the star's light, without the reflection and glow from the exoplanet.

"When the planet is orbiting the star, when it goes in front of the star, of course, you see the transit&mdashthe planet is blocking the light from the star," Koch says. "When the planet goes behind the star, the star is now blocking the light from that glowing, red planet, and that causes what's called an occultation."

Occultation is a much less pronounced phenomenon than a planetary transit, so Kepler's ability to track the occultation of a large planet indicates that it will be able to detect the transit of a smaller one.

The key observation from the new research is that the small dip in the HAT-P-7 b light curve when the planet passes behind its star "is roughly equivalent to the signal of an Earth-size planet when it passes in front of its parent star," says Paul Kalas, an astronomer at the University of California, Berkeley, who is not part of the Kepler team.

Detecting smaller, cooler planets is a long process. The only truly Earth-like planet we know of&mdashours&mdashtakes more than 150 times as long as HAT-P-7 b does to circle its star, so collecting data on similar planets across multiple orbits will take years.

Even the new hot Jupiters and other close-orbiting planets that Kepler finds will take extensive follow-up observations from the ground to confirm. (As Koch points out, a star's periodic dimming can be explained by one star eclipsing another in a binary star system.) Koch says he expects the first large exoplanets discovered by Kepler to begin rolling out early next year.

In the meantime, the early evidence that Kepler will be able to detect Earths is "absolutely convincing," Kalas says. "Essentially, they are offering a window into the scientific future: Kepler will soon detect an Earth-sized planet outside of the solar system."

Direct imaging

Directly imaging exoplanets is extremely challenging because of two effects. There is a very small brightness contrast between the host star and the planet and there is only a small angular separation of the planet from the host. In plain english, the star&aposs light will drown out any light from the planet because of us observing them from a distance much larger than their separation. To enable direct imaging both of these effects need to be minimised.

The low brightness contrast is usually addressed by using a coronagraph. A coronagraph is an instrument which attaches to the telescope to reduce the light from the star and hence increase the brightness contrast of nearby objects. Another device, called a starshade, is proposed which would be sent into space with the telescope and directly block the star light.

The small angular separation is addressed by using adaptive optics. Adaptive optics counteract the distortion of light due to the Earth&aposs atmosphere (atmospheric seeing). This correction is performed by using a mirror whose shape is modified in response to measurements from a bright guide star. Sending the telescope into space is an alternative solution but it is a more expensive solution. Even though these issues can be addressed and make direct imaging possible, direct imaging is still a rare form of detection.

Three exoplanets that are directly imaged. The planets orbit around a star located 120 light years away. Notice the dark space where the star (HR8799) is located, this removal is key to seeing the three planets.

The exoplanet transit method

When exoplanets pass in front of their host star (as seen from Earth), a portion of the start light is blocked out and a decrease in the photon flux is measured. Measuring the change in flux over time, allows for the creation of a light curve. Fitting models to the light curve, various characteristics such as orbital motions and atmospheric composition can be extracted. Both the size of the host star and the planet will determine the decrease in flux during the transit. The orbital distance between the exoplanet and its host star does not affect the transit depth due to the enormous distance from Earth.

The transit method is particularly useful for calculating the radius of an exoplanet. To first order (assuming the stellar disc is of uniform brightness, and neglecting any flux from the planet) the ratio of the observed change in flux, , to that of the stellar flux can be expressed as:

where and are the planetary and stellar radii respectively. As described below, limb-darkening will have an affect on the transit light curve, but to first order, the equation above holds.

Impact Parameter:

The total transit duration is heavily dependent on the impact parameter , which is defined as the sky-projected distance between the centre of the stellar disc and the centre of the planetary disc at conjunction* and is shown in Fig. 1**. Assuming a circular orbit the impact parameter is expressed as:

Fig. 1: The impact parameter varies from centre of stellar disk with being on the cusp of the disc.

Transit Duration:

The total transit duration, , defined as the time during which any part of the planet obscures the disc of the star, depends on how the planet transits the host star. If the exoplanet crosses the centre of the stellar disc (), the transit duration is the longest with signifying a shorter transit duration. With the help of Fig. 2 and using Pythagoras’s theorem, the length the planet has to travel across the disk of the star can be expressed as,

Fig. 2: Star-planet geometry showing the distance traversed by the planet, , impact parameter of the system, and the stellar and planetary radii, and respectively.

With the aid of Fig. 3, the exoplanet moves from to around its orbit, creating an angle (measured in radians) with respect to the centre of the host star.

Fig. 3: The orbital geometry of a transiting exoplanet system showing the projected distance travelled across the surface of the star, , between the points to and the angle this geometry forms, with the inclination, , and semi major axis, , shown.

With the assumption of a circular orbit, the distance around an entire orbit is , where a is the radius of the orbit. The arclength between points and is and the distance along a straight line between and is .

From the triangle formed by , and the centre of the star,

an expression of the full transit duration.

The Effects of Limb Darkening:

The effect caused by the stellar disk being brighter in the centre compared to the limb of the disk is called limb darkening. Photons emitted from the limb of the stellar disc at a certain atmospheric depth , follow a more oblique path through the stellar atmosphere compared to the photons emitted from the centre of the stellar disc as seen in Fig. 4. For the photons escaping the edge of the stellar disc, an optical depth of unity is reached at a higher altitude where the temperature is cooler () and the radiation is less intense causing the apparent darkening.

Fig. 4: Limb darkening of a star showing how the intensity and temperature diminishes as an observer looks towards the limb of the star.

The limb darkening effect is largest at short wavelengths where a highly rounded light curve is observed. For longer wavelengths the effect is less severe and the centre of the transit takes on a flatter shape (see Fig. 5).

Fig. 5: Two model light curves of the super-Earth GJ

1214b for observations at 5000 Å (blue) and (orange) using a tunable filter with a width of 12 Å. Observations at shorter wavelengths result in a deeper and narrower transit.

Orbital Inclination:

Radial velocity observations provide information about the minimum mass, of , assuming the stellar mass is known. To constrain the actual mass of an exoplanet, the orbital inclination, , has to be measured. This is done by fitting a analytical transit light curve to the data using the transit equation of cite. A transiting exoplanet which has an impact parameter or , will have a shorter transit duration, a shallower transit depth and longer ingress and egress times. This is seen in Fig. 6 where the effects of varying from to is shown.

Fig. 6: Inclination values ranging from to at intervals, with the shallowest light curve corresponding to .

* Conjunction: The point in the orbit where two objects are most closely aligned, as viewed from Earth
** The figures and derivations are adapted from “Transiting Exoplanets”, by Carole A. Haswell.


One example of a transit involves the motion of a planet between a terrestrial observer and the Sun. This can happen only with inferior planets, namely Mercury and Venus (see transit of Mercury and transit of Venus). However, because a transit is dependent on the point of observation, the Earth itself transits the Sun if observed from Mars. In the solar transit of the Moon captured during calibration of the STEREO B spacecraft's ultraviolet imaging, the Moon appears much smaller than it does when seen from Earth, because the spacecraft–Moon separation was several times greater than the Earth–Moon distance.

The term can also be used to describe the motion of a satellite across its parent planet, for instance one of the Galilean satellites (Io, Europa, Ganymede, Callisto) across Jupiter, as seen from Earth.

Although rare, cases where four bodies are lined up do happen. One of these events occurred on 27 June 1586, when Mercury transited the Sun as seen from Venus at the same time as a transit of Mercury from Saturn and a transit of Venus from Saturn. [ citation needed ]

Notable observations Edit

No missions were planned to coincide with the transit of Earth visible from Mars on 11 May 1984 and the Viking missions had been terminated a year previously. Consequently, the next opportunity to observe such an alignment will be in 2084.

On 21 December 2012, the Cassini–Huygens probe, in orbit around Saturn, observed the planet Venus transiting the Sun. [3]

On 3 June 2014, the Mars rover Curiosity observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides Earth. [4]

Mutual planetary transits Edit

In rare cases, one planet can pass in front of another. If the nearer planet appears smaller than the more distant one, the event is called a mutual planetary transit.

Transit of Venus as seen from Earth, 2012

Io transits across Jupiter as seen by Cassini spacecraft

Mercury transiting the Sun, seen from Curiosity rover on Mars (3 June 2014).

The Moon transiting in front of Earth, seen by Deep Space Climate Observatory on 4 August 2015.

The transit method can be used to discover exoplanets. As a planet eclipses/transits its host star it will block a portion of the light from the star. If the planet transits in-between the star and the observer the change in light can be measured to construct a light curve. Light curves are measured with a charged-coupled device. The light curve of a star can disclose several physical characteristics of the planet and star, such as density. Multiple transit events must be measured to determine the characteristics which tend to occur at regular intervals. Multiple planets orbiting the same host star can cause transit-timing variations (TTV). TTV is caused by the gravitational forces of all orbiting bodies acting upon each other. The probability of seeing a transit from Earth is low, however. The probability is given by the following equation.

where Rstar and Rplanet are the radius of the star and planet, respectively, and a is the semi-major axis. Because of the low probability of a transit in any specific system, large selections of the sky must be regularly observed in order to see a transit. Hot Jupiters are more likely to be seen because of their larger radius and short semi-major axis. In order to find earth-sized planets, red dwarf stars are observed because of their small radius. Even though transiting has a low probability it has proven itself to be a good technique for discovering exoplanets.

In recent years, the discovery of extrasolar planets has prompted interest in the possibility of detecting their transits across their own stellar primaries. HD 209458b was the first such transiting planet to be detected.

The transit of celestial objects is one of the few key phenomena used today for the study of exoplanetary systems. Today, transit photometry is the leading form of exoplanet discovery. [5] As an exoplanet moves in front of its host star there is a dimming in the luminosity of the host star that can be measured. [6] Larger planets make the dip in luminosity more noticeable and easier to detect. Followup observations using other methods are often carried out to ensure it is a planet.

There are currently (December 2018) 2345 planets confirmed with Kepler light curves for stellar host. [7]

During a transit there are four "contacts", when the circumference of the small circle (small body disk) touches the circumference of the large circle (large body disk) at a single point. Historically, measuring the precise time of each point of contact was one of the most accurate ways to determine the positions of astronomical bodies. The contacts happen in the following order:

  • First contact: the smaller body is entirely outside the larger body, moving inward ("exterior ingress")
  • Second contact: the smaller body is entirely inside the larger body, moving further inward ("interior ingress")
  • Third contact: the smaller body is entirely inside the larger body, moving outward ("interior egress")
  • Fourth contact: the smaller body is entirely outside the larger body, moving outward ("exterior egress") [8]

A fifth named point is that of greatest transit, when the apparent centers of the two bodies are nearest to each other, halfway through the transit. [8]

Since transit photometry allows for scanning large celestial areas with a simple procedure, it has been the most popular and successful form of finding exoplanets in the past decade and includes many projects, some of which have already been retired, others in use today, and some in progress of being planned and created. The most successful projects include HATNet, KELT, Kepler, and WASP, and some new and developmental stage missions such as TESS, HATPI, and others which can be found among the List of Exoplanet Search Projects.

HATNet Edit

HATNet Project is a set of northern telescopes in Fred Lawrence Whipple Observatory, Arizona and Mauna Kea Observatories, HI, and southern telescopes around the globe, in Africa, Australia, and South America, under the HATSouth branch of the project. [9] These are small aperture telescopes, just like KELT, and look at a wide field which allows them to scan a large area of the sky for possible transiting planets. I addition, their multitude and spread around the world allows for 24/7 observation of the sky so that more short-period transits can be caught. [10]

A third sub-project, HATPI, is currently under construction and will survey most of the night sky seen from its location in Chile. [11]


KELT is a terrestrial telescope mission designed to search for transiting systems of planets of magnitude 8<M<10. It began operation in October 2004 in Winer Observatory and has a southern companion telescope added in 2009. [12] KELT North observes "26-degree wide strip of sky that is overhead from North America during the year", while KELT South observes single target areas of the size 26 by 26 degrees. Both telescopes can detect and identify transit events as small as a 1% flux dip, which allows for detection of planetary systems similar to those in our planetary system. [13] [14]

Kepler / K2 Edit

The Kepler satellite served the Kepler mission between 7 March 2009 and 11 May 2013, where it observed one part of the sky in search of transiting planets within a 115 square degrees of the sky around the Cygnus, Lyra, and Draco constellations. [15] After that, the satellite continued operating until 15 November 2018, this time changing its field along the ecliptic to a new area roughly every 75 days due to reaction wheel failure. [16]


TESS was launched on 18 April 2018, and is planned to survey most of the sky by observing it strips defined along the right ascension lines for 27 days each. Each area surveyed is 27 by 90 degrees. Because of the positioning of sections, the area near TESS's rotational axis will be surveyed for up to 1 year, allowing for the identification of planetary systems with longer orbital periods.

Dip-Detection in the Kepler Data

Following the data release, a slew of Kepler papers went up on astro-ph this evening. In my previous post, I went straight to the numbers, but here I’ll discuss the Kepler mission and data in some depth, which I think will generally be useful for understanding current and future Kepler results.

The figures I’ve included are from a Nature paper by Jack J. Lissauer (NASA Ames) that was the focus of the NASA press release. This multiplanet system with 6 new planets all orbiting a star labeled “Kepler-11.” (The planets are then labeled with letters, in this case b-g. The first planet found is called [starname]b and so on. In this case, all 6 planets were detected simultaneously so the authors have labeled them from inside to out, e.g. Kepler-11b is the innermost planet and Kepler-11g is the outermost.)

The planets ranging in size from 1.97 to 4.52 Earth-radii, which are believed to be coplanar to within

1 degree. All but the outer most would be within Mercury’s 0.39 AU orbit, making it the most tightly packed planetary system discovered. That’s just the basics. It’s quite an interesting system and has been talked about a lot, so I’m just going to direct anyone interested to Phil Plait’s Bad Astronomy post about it.

  • Title: Characteristics of planetary candidates observed byKepler, II: Analysis of the first four months of dataAuthor: William J. Borucki et al.
  • First Author’s Institution: NASA Ames Research Center

Sample transit light curves from the Kepler-11 system. (Click to make bigger)

The goal of the Kepler mission is to figure out how many Earth-analogs (similar size and climate) are out there. There are several ways to detect exoplanets, but Kepler uses the transit method: when a planet passes between us and its host star, it causes a dip (

1%) in the star’s observed brightness (flux). This dip has a characteristic inverted top-hat shape and the light curve can be used to determine the planet’s orbital period and radius. With radial velocity data, the planet’s mass and thus its density can be determined (the radial velocity method relies on observing Doppler shifts in the star’s spectra as it “wobbles” the wobble is the result of the star orbiting around the star/planet center of mass.)

Unfortunately, the planetary system has to be in an opportune alignment for us to see a transit so we don’t expect to see transiting planets around most stars. To get around this, Kepler is monitoring

150,000 stars similar to the Sun over a period of 3 years. For now, we’ll have to settle for the results based on the first four months of data. In the figure to the left, you can see examples of 6 different transit events, one for each of the Kepler 11 planets. These are plots of flux (normalized to the out-of-transit flux) against time.

One important aspect of this research is the identification of false positives: objects that display light curves consistent with a planetary transit but are in fact something else. False positives generally fall into two categories. The potential transit signature could just be noise in the data, or it could result from a different astrophysical event. Eclipsing binaries, variable stars or multi-star systems could be selected as potential transits by Kepler’s planet finding robot (which they refer to as the Transiting Planet Search pipeline). Nathan discussed the effect of stellar variability on exoplanet detection in a previous post. Tests and further observations are done to weed out these interlopers. The false positive rate in the Kepler candidate list could be as low as 5%, although uncertainties, such as what Kepler can easily identify as not being a transit signature, are a factor.

Below, I’ve included the Kepler 11 data. The top panel is the raw data and the bottom is with the trends removed. There are some gaps in the data, but you can see narrow dips that mark transits (the few points scattered about are noise) the transits of each planet are marked in differently-colored dots along the bottom. Of course, because we typically only detect 1 or 2 planets in a system rather than 6, there are usually there are far fewer dips. This might give you some idea as to the difficulty of validating transiting planets note especially the scale on the flux!

Raw (top) and detrended (bottom) light curve of the Kepler-11 system. Dips mark transit events each transit is marked by a dot and each planet is labeled in a different color. Note what a small percentage of the total flux each dip is.

In addition to the possible false positives, there are several other problems with drawing statistical conclusions from the Kepler data set right now: the particular stars selected for study, noise and methods of analysis all bias the results and certain planets (large planets at small orbital radii passing in front of a dim star) are easier to detect than others. All of these effects on planet detectability need to be well-understood and included in the statistical analysis. The survey is also not complete: the faintest stars and smallest candidates especially need more data. Additionally, current study is restricted to those planets with periods less than 125 days (planets out there with longer periods will have only transited once or not at all!).

That’s not to say the results aren’t very interesting! As you may have read in my previous post, Kepler has now found 1,235 planet candidates. They range in size from about Earth-sized to larger than Jupiter. About half of these candidates are the size of Neptune, but 68 are similar in size to the Earth the size distribution peaks at 2-3 Earth-radii and declines at larger sizes. Among them are more than 150 candidate multiple transiting planet systems. A variety of methods help to validate such candidates and constrain their orbits and masses. In turn, these can tell us about the formation and evolution of planetary systems, especially planet migration.

Unistellar Citizen Science

Highlight: Results from eVscope observations of exoplanet HD 189733b

On November 6, 2020 Unistellar citizen astronomers observed exoplanet HD 189733b pass in front of (or transit across) its star!

HD 189733b is a hot, Jupiter-sized exoplanet that orbits its star every 2.2 days. It is 13% more massive than Jupiter and its winds blow 7 times the speed of sound!

Interestingly, it was the first exoplanet to have its color measured! Its blue color doesn’t come from oceans of water on its surface but possibly from a hazy atmosphere with clouds full of tiny glass shards. Read more about these observations of this “true blue planet.”

“By combining 3 citizen astronomers’ eVscope observations from across Europe, we timed HD 189733b crossing its star to within a couple of minutes, 63 light-years from Earth!,” said Tom Esposito, lead exoplanet astronomer with the SETI Institute & the Unistellar Exoplanet Team.

Transit light curves like this one are used to visualize an exoplanet transit from collected data. This light curve shows the transit of HD 189733b as it passed in front of its star, which caused the flux (brightness) of its host star to dim and then return to normal. The depth of the dip can tell us about the planet’s size as compared to the star. The gray circles and blue squares on the large graph indicate the brightness of the star over time, which were compared to astronomical models, indicated by the red line. This shows that the star’s brightness dimmed by 2.8% (the dip along the plot’s vertical axis) as HD 189733b crossed the star. The smaller plot below shows residuals, or how well the data matched the astronomical models.

The combined observations were taken by Unistellar citizen astronomers Mario Billiani (Austria), Stephan Abel (Germany), and Julien de Lambilly (Switzerland).

Their combined observations were consistent with previous transit observations from 10 different telescopes taken during 2005 to 2006, which is pretty impressive for a 4.5-inch telescope that can take observations right from your backyard!

Esposito added, “Also, combining the measurements led to a more precise measurement of the transit time than using individual measurements, which shows great promise for future observations with the growing Unistellar Network!”

“It’s like in the old cartoon: with your powers combined… we are Captain Planet!”

Unistellar works closely with the SETI Institute, thanks to their partnership, which includes a great team of exoplanet scientists.

Many thanks to the SETI Institute & the Unistellar Exoplanet Team: Tom Esposito, Dan Peluso, and Arin Avsar for planning this observation, data reduction, and sharing these great results with us!

Testimonials from Unistellar Citizen Astronomers

Mario Billiani, Unistellar Citizen Astronomer from Austria

“This was my first successful detection of an exoplanet with my eVscope so this campaign result is of particular value to me. I also appreciate that we could combine the light curve with two other simultaneous observations to improve the data.”

Stephan Abel, Unistellar Citizen Astronomer from Germany

“The observation of the HD 189733 b transit was the first opportunity my eVscope had to be used in a Unistellar citizen science project. My first problem was to find a suitable observation site. Light pollution in the southwest of Germany is generally high, and I live near Koblenz, where this is even more pronounced.

So I chose a viewing platform for hikers about 20 km from my apartment, near the village of Rüscheid.

This is located in a field, so I had to carry my eVscope about 500m there, but thanks to my backpack and low weight, it was no problem!

The weather was cool and windy, I was very happy that I had warm clothes and hot tea with me. Also were two horses in the paddock, eyeing me curiously. Otherwise there were no spectators. I am fascinated by the sight of starry skies every time, even if a lot cannot be seen with the eyes in Bortle class 4 area.

It was pure luck that I bought the eVscope in August. I hadn’t heard of it before, but I wanted to get a telescope again.

In my youth I owned a 6 ” Newtonian telescope, an entry-level model, difficult to handle.

I was immediately convinced of the design of the eVscope, as it is easy to use and easy to transport in combination with digital optics which are crucial for hobby astronomy.”

Originally translated from German:

“Die Beobachtung des Transit von HD 189733 b war die erste Gelegenheit mein EVScope im Citizensience Projekt von Unistellar zu nutzen. Mein erstes Problem bestand darin einen geeigneten Beobachtungsort zu finden. Im Südwesten Deutschland’s ist die Lichtverschmutzung allgemein hoch, und ich wohne in der Nähe von Koblenz, wo diese sich noch störender bemerkbar macht.

Ich wählte daher eine Aussichtsplattform für Wanderer etwas 20 km von meiner Wohnung, bei dem Dorf Rüscheid.

Diese ist auf einem Acker gelegen, daher musste ich mein EVScope noch etwa 500m dahin tragen, aber Dank Rucksack und kleinem Gewicht kein Problem!

Das Wetter war kühl und windig, ich war sehr froh, dass ich warme Kleidung und heissen Tee dabei hatte. Auch waren zwei Pferde auf der Koppel, und beäugten mich neugierig. Ansonsten gab es keine Zuschauer. Der Anblick des Sternenhimmels fansziniert mich jedes mal, auch wenn bei Bortle Klasse 4 vieles mit dem Auge nicht zu sehen ist.

Es war reiner Zufall, dass ich das mir das EVScope im Auguste kaufte. Ich hatte vorher nichts davon gehört, wollte mir aber ein Teleskop wieder zulegen.

In der Jugend besaß ich ein 6″ Newton Fernrohr, ein Einsteigermodell, schwer zu händeln.

Von dem Design des EVScope war ich sofort überzeugt, da mir einfache Bedienbarkeit und gute Transportfähigkeit in Kombination mit der digitalen Optik für die Hobbyastronomie ausschlaggebend sind.”

Julien de Lambilly, Unistellar Citizen Astronomer from Switzerland

“The HD 189733b campaign was particularly interesting to me because they wanted to compare the eVscope to research telescopes. I wanted to know how valuable our data could be!

So on November 6, I went to a mountain pass, le Col du Marchairuz at an elevation of 1389m in Switzerland to observe the HD 189733b transit. The sky was clear, Bortle class 4 and there was a lot of humidity, but luckily no wind at all. The temperature went from 4 to 2 degrees Celsius by the end.

I was a bit worried, because this was the first time I’d observed an exoplanet transit on a bright star like HD 189733.

I don’t know why I was worried anyway, Tom Esposito did the maths and told me it would fit just right!

On November 25 we discovered the results on the Unistellar slack channel. It was the first time the exoplanet experts from Unistellar were able to combine the data from different observers! And the light curve looked beautiful and indeed more precise than my own alone. This makes even more sense to work as a community! I’m proud to have participated! Special thanks to Dan Peluso and Tom Esposito for providing the opportunity!”

Positive Detections

The following is a list of positive detections of exoplanet targets with the Unistellar Citizen Science Network:

Amateur Detects Exoplanet Transit

By: Robert Naeye September 3, 2004 0

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This artist conception depicts a Jupiter-size planet transiting its host star at a close distance. Astronomers know of six stars that are transited by exoplanets two have been detected by amateurs, including the recently discovered planet TrES-1.

S&T illustration by Steven A. Simpson.

the discovery of TrES-1, an extrasolar planet that transits its host star. Just 8 days later, an amateur astronomer from Landen, Belgium detected a transit of the same planet. The discovery highlights the growing capabilities of amateur astronomers and proves that amateur astronomers can, in principle, discover an exoplanet by the transit method.

Tonny Vanmunster used a Celestron C-14 telescope and an SBIG ST-7XME CCD camera (without filters) at his private CBA Belgium Observatory to detect the TrES-1 transit. The telescope rested on an Astro-Physics AP1200 GTO mount. The planet began crossing the star's disk at 21 hours, 13 minutes Universal Time on September 1st, just when the transit was predicted to commence. The event lasted about 3 hours and ended right on cue. The star's brightness dipped by about 0.03 magnitude during the transit, or roughly 3 percent. Using software he wrote himself, Vanmunster monitored the progress of the transit in real time on his computer.

Belgian amateur astronomer Tonny Vanmunster obtained this light curve of a faint star in Lyra. The 0.03-magnitude dip in the star's brightness is caused by a Jupiter-size planet crossing in front of the star's disk, an event known as a transit. The quality of data decreased near the end of the observing session because the star sunk close to the horizon. Click on the image to view a larger image.

Courtesy Tonny Vanmunster.

Belgian amateur astronomer Tonny Vanmunster, seen here in his private observatory, detected the exoplanet TrES-1 as it transited its host star. He has also detected the transit of another exoplanet, HD 209458b. Besides his exoplanet work, he observes cataclysmic variables and supernovae.

Courtesy Tonny Vanmunster.

Tonny Vanmunster used this Celestron C14 telescope to make the first amateur detection of the transiting exoplanet TrES-1.


Planet Hunters is a citizen science project that makes it possible for anyone to sieve through data taken by the NASA Kepler space mission. The Kepler spacecraft takes brightness measurements, or "light curves," of over 150,000 stars every 30 minutes. People can then hunt for planets by looking for a brief dip in brightness that occurs when a planet passes in front of the star.

Join the search at, or read on to learn more.

Our Challenge

NASA's Kepler spacecraft is one of the most powerful tools in the hunt for extrasolar planets. The Kepler team's computers are sifting through the data, but we at Planet Hunters are betting that there will be planets which can only be found via the remarkable human ability for pattern recognition.

This is a gamble, a bet if you will, on the ability of humans to beat machines just occasionally. It may be that no new planets are found or that computers have the job down to a fine art. And yet, it's just possible that you might be the first to know that a star somewhere out there in the Milky Way has a companion, just as our Sun does. Fancy giving it a try?

The Kepler Public Data

On March 2009, the NASA Kepler mission was launched with the goal of using the transit technique to detect exoplanets: terrestrial and larger planets orbiting other stars. With this method, planets that pass in front of their host stars block out some of the starlight causing the star to dim slightly for a few hours. The Kepler spacecraft stares at a field of stars in the Cygnus constellation and records the brightness of those stars every thirty minutes to search for transiting planets.

The time series of brightness measurements for a star is called a light curve. The Kepler spacecraft beams data for more than 150,000 stars to Earth at regular intervals. With every download of data, the time baseline of the light curves is extended.

The project's Principal Investigator, Bill Borucki, began planning the Kepler mission in the mid-1980's and his team has been hard at work for more than a decade. To reward them for this hard work, the Kepler team has advanced access to the light curves. We at Planet Hunters are not part of the NASA Kepler team. However, NASA is releasing light curves into the public archive to encourage broader participation and we think that the public can play an important as our scientific partners in this latest Zooniverse project.

  • A special orientation of the orbit is required. Because the technique looks for a dimming in the brightness of the star, all of the planets with orbits that don’t pass between the star and our line of sight will be missed.
  • Close-in planets are easier to detect. A transit event only occurs once per orbit planets that are closer to their host stars race around their orbits faster than planets that orbit at larger distances. To confidently detect a transit, at least three dips in the brightness (i.e., three transit events) must occur. Thus, a planet that orbits in one year, like the Earth, requires three years of data for detection, while planets that orbit in ten days can be detected with just thirty days of data.
  • Larger planets are easier to detect. The bigger the planet, the more starlight it blocks out. The Kepler mission measures the brightness of stars with such incredible precision that it is sensitive enough to detect transits of planets approaching the size of Earth.
  • Planets may be harder to detect against a variable brightness background. This turns out to be less of a problem than one might think. When we look at a light curve, we're seeing how the brightness changes with time. In the Figure below, there are starspots in addition to the transiting planet (lower left spot on the star). However, starspots rotate with the star and cause relatively slow changes in the brightness of the star. Transiting planets cross the star in hours and cause quick dips in the brightness of the star. Look below to see the difference (left image). Spots cause most of the smooth and slowly varying brightness and we're learning that many stars have much larger spots than the Sun.

Humans vs. Machines

The Kepler team has been developing computer algorithms to analyze light curve data because it is not possible for them to visually inspect every light curve. While we expect computer programs to robustly identify things that they are trained to find, we are betting that there will be a number of surprises in the data that the computer algorithms will miss.

The human brain is particularly good at discerning patterns or aberrations and experiments have shown that when many people work together, the collective wisdom of the crowds can be better than an expert. Planet Hunters is an online experiment that taps into the power of human pattern recognition. Participants are partners with our science team, who will analyze group assessments, obtain follow up observations at the telescope to understand the new classification schemes for different families of light curves, identify oddities, and verify transit signals.

Planet Hunters: Sorting the Light Curves

You will be looking at changes in star brightness at a level that has never before been seen. As you sort through the light curves, you will notice different patterns. In many cases, the data scatters in a relatively flat band of points, like the cases shown in Figure 1. Most of this scatter is simply the inevitable noise that comes with any measurement. Other light curves, like those in Figure 2, are obviously variable with time. We think that most of the variability (on timescales of hours to days) is caused by starspots or pulsations. Having Planet Hunters sort families of similar light curves is part of the important scientific research.

Figure 1. Even precise measurements are not exactly perfect or reproducible and cause low-level scatter in the data. There is no visible pattern, just white noise, and so these light curves are best categorized with the middle 'quiet' icon.

Figure 2. These light curves should be tagged with the “variable” icon then you will be asked to decide if the curve is pulsating with one cycle (like the top left curve) or regular (like the top right curve) or irregular (like the bottom curves).

It is challenging for computer algorithms to classify variable patterns so participants are making a particularly important scientific contribution in this step. We will obtain follow-up data at the telescope to understand the underlying mechanisms for these different families of variable curves and to confirm transit candidates.

Planet Hunters: Flagging Transit Events

However, the real treasure hunt is for transiting planets and these present as a relatively sharp dip in brightness in the light curve (Figure 3). A transit could appear in either a quiet or a variable curve. Indeed, it will be more difficult for computer algorithms to find transits imposed on the variable light curves, so we hope that Planet Hunters will pay particular attention to these.

Figure 3. After sorting the light curves as quiet or variable, Planet Hunters will be asked if there are any possible transits in the data. If low points are seen, then answer 'yes' and click on the icon to create a box that can be positioned over the transit features.

The size of the planet is reflected in the depth of the transit points. Earth-sized planets will exhibit a dip in brightness that is buried in the noise of the quiet light curves in Figure 1. The transit events in Figure 3 are for planets that are several times the radius of the Earth.

The time it takes a planet to complete one orbit is called the orbital period. For transiting planets, this can be determined by counting the number of days from one transit to the next. The examples in Figure 3 are fairly obvious. Planets in longer period orbits will be more challenging to detect, both for humans and for computers because a transit will not appear in every 30-day set of light curve data. Just because you don’t see a transit in the first block of data doesn't mean that there won’t be a transit in another set!

Large planets with short orbital periods are the easiest ones to detect. The most challenging detections will be small planets with long orbital periods. These will require patience and care, but are the real treasures in the Kepler data!

I See a Transit!

What happens if Planet Hunters discover a possible transiting planet? We maintain a list of transiting planets that the Kepler team announces, so the first thing that will happen is that we will check that list. If the flagged transit event is for a star that the Kepler team are already keeping an eye on, we'll let you know. If this event has not been identified and several Planet Hunters are flagging the same data, the science team will investigate. If this appears to be a new discovery, then we will follow up to obtain spectroscopic data using the Keck telescope in Hawaii. If the transit candidate passes all of the screening tests, the result will be submitted for publication. Planet Hunters who discover new transiting planets will be included as co-authors on our papers.

Experimental Data: Faking the Transits

To better understand what types of planets are detected or missed by Planet Hunters, we have created a small number of test cases for some of the light curves that participants will classify. These test cases contain fake transit events and are critical for determining the statistical completeness for planets as a function of size (depth of the transit event) and orbital period (number of transits). After participants classify a data set with a fake transit event, a message will appear notifying them that this was a test case, and the fake points will be highlighted.

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Vol 325, Issue 5941
07 August 2009

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By W. J. Borucki , D. Koch , J. Jenkins , D. Sasselov , R. Gilliland , N. Batalha , D. W. Latham , D. Caldwell , G. Basri , T. Brown , J. Christensen-Dalsgaard , W. D. Cochran , E. DeVore , E. Dunham , A. K. Dupree , T. Gautier , J. Geary , A. Gould , S. Howell , H. Kjeldsen , J. Lissauer , G. Marcy , S. Meibom , D. Morrison , J. Tarter

The Kepler mission is performing at the level required to detect Earth-size planets orbiting solar-type stars.