All sky map of exoplanet host stars: Surveys (exoplanet.eu)

There are many different surveys looking for new extra-solar planets. Right now the most successful technique is the transit method and there are many programs using this method. In this all sky map I was trying to show a few of these surveys which look at many stars, monitor their brightness, and try to find exoplanets in the stars' light curves.

Probably the most well-know survey is Kepler; it's the instrument that found the most planets until now. However, it looked at a very limited area of the sky. You can see it on the left side of the map where the large number of black dots are.

CoRoT was another space-based mission which was very successful; however, it did not nearly find as many exoplanets as Kepler. Its observing strategy was a bit different. CoRoT did not stare at one area in the sky all the time but switched its field of view every half year. One time it looked in the direction of the center of the Milky Way, which are the purple squares on the left side, and the next time it looked at an area in the opposite direction. These so-called 'anti-center' runs can be seen in the center of the map.

Also very successful is WASP. The nice thing about this survey is that (1) it observes from the ground and (2) it looks (almost) everywhere in the sky. The 104 planets I got from exoplanet.eu (including SuperWASP) are really quite a number.

There are lots of different missions listed in the legend of the map and I do not want to go into detail on everyone of them. It is not a complete list of programs but rather what I could made out of the information provided by exoplanet.eu. Unfortunately, it is kinda hard to find which planet was detected by which survey because this is not listed there. So the host stars (and planets) I plot here are named after the program they were found with - and this way I could plot them. However, this is not true for every planet found and also not true for every survey.

This is especially disappointing for the planets found with the RV method. I would have liked to plot all the RV planets coming from, e.g., HARPS, which is an amazingly successful instrument, but I could not easily collect the necessary information. The host stars usually keep their name because they are bright, well-known objects (with HD or HR numbers). So it's not possible for me to get the survey from the name alone.

After all, this map is supposed to give an impression of which transit surveys there are and how they observe. It's not meant to be complete. Furthermore, the detection method listed in the database is not always 'transit'. This is because some systems are re-observed in RV to get the mass of the planet. Others, e.g., OGLE are no transit survey, although they do observe light curves. It can by chance also find transits but OGLE is actually looking for the signatures of microlensing events.

All sky map of exoplanet host stars: Multi-planet systems (exoplanet.eu)

With this post I continue my series on exoplanet host star all sky maps. What you see is basically the same thing I showed two posts ago. This time, however, I choose the color to represent the number of exoplanets in the system. So the symbol tells you which method was used to detect the system and the color tells you how many planets are know right now in this system (August 3, 2015, exoplanet.eu). The 740 lightyellow-colored host stars have only one planet, the one black circle in the Kepler field of view (Kepler-90 alias KOI-351) is the only exoplanet system with seven planets - all transiting the host star.

The Kepler field of view is so crowded that I include a zoom in on this region; it's the figure on the left side. The only seven planet system is on the top right of the Kepler FOV whereas the only transiting six planet system (Kepler-11) is pretty much at the bottom. In the all sky map you can see three other six planet systems, however, these where detected with the RV method.

Exoplanet host stars: The Kepler field of view (exoplanet.eu)

This is an addition to the post before. The plot shows the Kepler field of view in detail, all symbols and colors stay the same. This is the region in the sky from which we - by far - know the most planets around stars. Until July 23, 2015, Kepler has discovered 1879 confirmed exoplanets around 471 stars (according to exoplanetarchive.ipac.caltech.edu).

All sky map of exoplanet host stars (exoplanet.eu)

Today I'd like to show something not directly connected to the last couple of posts. This is an all sky map of exoplanet host stars. Different colors/symbols indicate the method used to detect the planet. The data was taken from exoplanet.eu. Actually, I tried to plot all planetary systems know today, which according to exoplanet.eu should be 1228, but for some reason I am missing three in the transit method.

The underlying magenta colored points are about one million stars from the Tycho catalog (ESA Hipparcos satellite). The map is an aitoff projection of the sky in galactic coordinates and the Milky Way lies at the equator; this is why most of the stars are located there.

Most of the planets are detected with the transit method. However, the majority is located in the small area crowded with black dots - the Kepler field of view. When we talk about statistics of transiting exoplanets we are actually talking about a small part in the sky and not the entire sky. Astrophysicists just assume that it should be the same everywhere. The other black dots come from different surveys, e.g., CoRoT or WASP.

The picture is different when looking at planets detected with the radial velocity (RV) method. They seem to be more uniformly distributed. It's the second most successful technique with 454 stars having planets around them.

At least according to exoplanet.eu, so far there is only one astrometry planet and it is very close to the Kepler FOV - although it's not in it. The star is HD 176051 b. So far it seems to be too difficult to detect planets with this method, but it is expected that with GAIA there will be many planets coming from this technique.

Imaging seems pretty much located to certain areas, avoiding the plane of the Milky Way as much as possible. This is probably a good idea since background stars that coincidentally stand close to the potential host stars make it more difficult to find planets - or might even be misinterpreted as bodies belonging to the system.

All microlensing planets come from a limited area in the direction of the center of the Milky Way, which in this map is at the edge. This illustrates nicely that microlensing really samples a quite different population of stars. All other techniques try to go away from the galactic center (and even the galactic plane) and only find exoplanets rather close to the Sun. The most distant microlensing planet, however, is about 25000 light years away - virtually in the center of the Milky Way.

The transit timing variation (TTV) planets are all in the Kepler FOV. For this method one needs good light curves and a long cadence to cover many transits, and this is what Kepler does best. According to exoplanet.eu there are only four TTV planets - which I do not think is true; there have to be much more. exoplanets.org says it's more like 60 (including some pulsar planets), which I believe is closer to the real number.

Finally, we have the planets around pulsars. There are a few in the Kepler FOV, but otherwise I do not really see a system there. I think it is nice to notice that PSR 1257 12 b is the star with the first exoplanet detection in the year 1992 - and it actually is a three-planet system! I think is has to be one of the two triangles in the upper right. Usually, people cite 51 Peg b as the first exoplanet - it was the first around a solar-type star. I guess pulsars are just too different ...

Transits: Limb darkening - HD 209458 b in different colors (HST)

In the previous post I presented theoretical transits of how the exoplanet HD 209458 b should look like in different colors. Now I will show how its transits really look like.

On the left you see observations of the Hubble Space Teleskope (HST) from 320 to 970 nm. The last transit, which is much nosier, is not from HST but Spitzer - a NASA space misson for infrared observations.

Actually, HST observed many spectra of the star covering a full transit. By averaging over different parts of the spectrum you get the transit in a certain color. The central wavelength of the interval over which was averaged is given on the right side of each transit. The colors range from the ultraviolet to the near-infrared.

The Spitzer data is special because it lies at a much higher wavelength in the far-infrared. There the star should have virtually no limb darkening anymore and the transit has a box shape. However, you cannot see that very well because it is much more difficult to get precise brightness measurements for these wavelengths. The star is much fainter there and, thus, the noise is much higher. Also the two instruments are pretty different which leads to non-equal measurement errors.

In the last post I showed what we should expect from theory - and the observations agree nicely with it. In UV and blue the shape of the transit is much rounder than for longer wavelength. Indeed, limb darkening is stronger for shorter wavelengths.

It is actually pretty hard to measure the limb darkening of stars; they are just too far away to spatially resolve them. Analyzing exoplanetary transits is one of the best methods to verify whether the theoretical predictions are really correct. And so far the models seem to work pretty well - although in detail observations and theory are not so easy to compare. After all, the exact shape of the transit depends on a lot of different things.

Transits: Limb darkening in theory

Now let us take a look at limb darkening in transits. In the beginning I will start with theoretical transits - transit lightcurves generated on the computer. This makes explanations a bit easier, we will come to real data soon enough.

For demonstration purposes I chose one of the best known exoplanets today: HD 209458 b. In 1999 this was the first transiting planet ever observed (Charbonneau 2000), a true milestone in the history of astrophysics. It is a huge planet, about 1.3 times the radius of Jupiter but only 70 % of its mass. It is a good planet to start with because its large size results in a deep transit.

The figure on the left side shows simulated transits of HD 209458 b for different wavelengths. This is interesting because the limb darkening of the star depends on the color you are looking at. So in different colors the transit shape will be different. I added a bit of noise to the lightcurve to make it look more like a real observation. The noise is roughly consistent with the uncertainty of the Hubble Space Telescope data of this planet I will show in the next post.

All transits are normalized meaning that the (average) brightness outside of the transit is one. However, if I plot them all over each other we would not see them very well anymore; so transits observed at longer wavelengths are shifted downwards. The corresponding wavelength (in nm) is given on the right side of each transit lightcurve. On the very right it is indicated to which wavelength regime (UV, visible, IR) this wavelength belongs. Keep in mind that visible light goes from 380 to 750 nm.

The x-axis shows you the time of the observation of each data point in hours from transit center; the transit duration of HD 209458 b is about three hours.

What can we learn from this figure? Due to the limb darkening the transit shape is round for blue colors (shorter wavelengths) and gets more box-shaped for red colors (longer wavelengths). When going further into the infrared, the transit looses its round shape completely because there is (almost) no limb darkening of the stellar disk anymore. In real data this would be nice because dealing with limb darkening can be a nuisance; you just have to deal with a few parameters less you do not really know for sure. However, it is not that easy to obtain high-quality transit lightcurves in the infrared - even for the best instruments scientist have right now. This is what I will show you in the next post.

Addendum: A more technical note at the end. On the left side of the transits in the figure letters are written: U, B, V, R, I (and FIR). These letters indicate the photometric bands (Johnson filters) used to simulate the transit lightcurves which are important to know the theoretically expected limb darkening. If you know your star and the photometric filter, you can calculate how the limb darkening should be. Here I use the non-parametric limb darkening coefficients for the Johnson filters for a Sun-like star published in this paper.
FIR is not a real filter; it stands for far-infrared. I just assumed at this long wavelength the limb darkening is zero. This way we have a comparison how the transit would look like if there is no limb darkening at all.

Limb darkening: SDO 17.1 nm

Finally, I come to the point where limb darkening actually is not 'darkening' anymore. I should rather say limb brightening here. In the figure you can see the brightness across the solar disk again normalized to the value at disk center. However, this time you see that the Sun is brighter on the edges than it is in the center.

I generated this plot from SDO images like the one on the left. They show the Sun in the extreme ultraviolet at a wavelength of 17.1 nm. At these short wavelengths the disk is very inhomogeneous and you see a lot of loop structures which show the magnetic field of the Sun. Like in the last blog on SDO 1600, this is not the photosphere we are seeing; it's not even really the chromosphere anymore, but more like layers above that. Here we start to see the corona of the Sun.

In this high-energetic wavelength regime the Sun has no sharp edge. The top panel shows how the brightness goes down after the peak at the edge, but it does not drop down to zero like in the other images I showed in previous posts. The bright loops extend far into outer layers of the Sun and you still get a significant amount of light from there.

Magnetic lines are connected to the activity of a star. The more active a star is, the more of these bright inhomogeneous structures it should have in its outer atmospheric layers. Like spots these structures rotate with the star and change all the time.

For wavelengths showing limb brightening, exoplanet transits would look very different for what we see in the visual or infrared. Transits can change significantly from one to the other because the magnetic structures change. Also, contrary to the visual wavelengths, the transits are deeper at the edges than in the center. However, transits at these short wavelengths (far-UV, EUV, or even X-rays) are very hard to detect.