D-1: Just Over The Horizon

The following is Murmeldjuret’s final guest column in this series. It was originally published on the Stellaris forums.


Now that we are getting close to release, you know what would really suck? Time slowing down or even stopping.

Gravity slows time down, very marginably. The GPS satellites experience time slightly faster than we do at the surface. It is only a few microseconds each day. The closer you are to a densegravitational source, the more spacetime is curved, and the slower time passes. If you follow this to its extreme, you can with a gravitational singularity stop time as we know it.

(The black hole from Interstellar)

The most common singularity is what we call a black hole. We call it black because the most used description is that light can’t escape it. This is a simplification. Nothing escapes a black hole. The direction of out no longer exists. Just at the event horizon or surface of the black hole, the spacetime paths that light can take all are bent into leading further into the hole.

For an outside observer, anything falling in will slow down and be redshifted. The moment of it arriving at the event horizon, from the view of an outside observer, it will have stopped entirely. The redshift is now infinite as the time it will take the light to leave is infinite. That last light that never can leave is the event horizon. The event horizon is simply put a stitched together canvas of all the things that have fallen in. The surface is the last event of anything that has fallen in spread out for all future to witness. If a million years passed, that same event would be on the surface, still trying to send out the same light.

Now here is the weird bit. Black holes gain mass when things fall in. Yet nothing ever falls in, as time has stopped on the surface for everything outside the black hole. The weight is from all future events. So in a million years, its mass is felt outside, but its past is still stuck on the horizon.

So how does it look from the perspective of anyone falling in? For them time passes normally. It is like any gravitational free fall. Eventually the gravitational difference will be so great matter gets torn apart. Probably. For us on the outside, nothing happens in a black hole. It just happens to be a heavy collection of futures with a surface of pasts.

So what happens when to black holes merge?
This is a simulation of how the gravitational lensing would look like.
Oj287 is probably a supermassive binary black hole and it will merge sometime within 10000 years. PG 1302-102 is a very likely candidate for a supermassive binary black hole and if so it has already merged and the event will reach us in just shy of a million years. It will shine brighter than a supernova even though it is billions of lightyears away. We know that black holes merge, we have even detected the gravitational waves now, so even if black holes are still weird, logic defying things, we have found them.

We know what happens when two black holes collide from the outside. What happens on the inside? What happens when two events that do not occur collide? Does it even happen if nothing can observe it? What is even the inside of a black hole?

Quantum effects on the black hole surface create Hawking radiation, often called black hole evaporation. Entropy suggests black holes shrink, and it does indeed seem they can do so. Freeing up the events that have already happened as radiation. The universe truly is weird.


D-2: The Gift of Giving

This piece was written by Murmeldjuret as part of the Astroknowledge series and is reproduced here with their permission. It was originally published on the Stellaris forums.


Today I will be continuing The Beautiful Void’s astroknowledge series.

Right now you are emitting radiation. So are your walls. Don’t worry it is perfectly normal and won’t kill you. You should actually be glad the walls radiate back to you, if they wouldn’t you would start freezing. The same reason the walls radiate heat to you is the way we find the temperature of suns millions of lightyears away.

All light and heat is created when an electron loses potential energy. It is how lightbulbs, LEDs, x-ray cathodes, and forest fires emit light. Whenever an electron moves from a high energy state to a low energy state it emits the difference as electromagnetic radiation. Short jumps become low energy radio waves, and the longest jumps become gamma rays.

X-ray cathodes work by sending high velocity electrons at a target material, knocking out electrons from their orbit. Higher energy electrons fall into these holes, and emit X-rays while doing so.

Atoms and molecules also have kinetic, rotational, and vibrational energy levels. When they interact with each other, they change their internal energy levels. This is what we see as temperature. Hotter objects have more kinetic, rotational, and vibrational energy. Temperature is their ability to give off this energy to other things. When they touch another colder object, they will lose heat energy and the colder will gain that heat energy. Temperature equalizes because something that is better at giving than something else gives back will lose. This seems natural and is something humans notice quickly. Touching cold objects lowers your temperature, while nobody wants to touch hot coals because they love to give.

Heat has another way of transferring than simple touch, namely as radiation. The side facing a hot fire will become hotter than the side facing away, which is not due to the heat transfer via the air between. The vibrational energy can be transferred via photons, as well as normal matter. All things above absolute zero has vibrational energy and as soon as it interacts with another electron it will change its vibrational energy. Any loss here is emitted as electromagnetic radiation. Much of it is contained inside the object, but anything close to the surface has a fair chance to emit it out of the body.

This is then also related to temperature, or willingness to lose energy. The heat you radiate is absorbed by the walls and their radiation is absorbed by you. Heat radiation is for most parts marginal inside earth’s atmosphere. Air is a better conductor for heat than radiation. This is not the case in space. In space, all heat is exchanged through radiation.

The type of radiation is not dependent on the object, or the shape, or the substance. It is only dependent on temperature, the willingness to give energy. This is Planck’s Law, and describes how much and of what type of radiation an object emits. It is always shaped this way, as the total number of atoms is so incredibly large any oddities get marginalized.
We can from this estimate the peak temperature of emission, and it follows the very simple Wien displacement law. Wavelength = constant/Temperature. It is a good approximationexcept for really cold temperatures.

So what does this mean?
It means that whenever we look at an object in its spectrum, we can accurately give its exact temperature. Below is the sun:
We can say that the surface temperature is 5778K within a few degrees of error. Similarly, we cantake the temperature of any distant object that we can spectrally resolve. It also shows why hot suns are blue, as their peak is to the left of the visible spectrum, and why cold stars are red, as their peak is to the right of the visible spectrum.

For those who wonder why we often call it black-body radiation, it is because the actual formula includes an emissiveness term, as radiation from object to medium isn’t perfect. Low emissiveness works like heat mirrors. The heat is never taken up by the object to be re-emitted. If emissiveness is max, it follows the curve exactly and this is called black-body radiation. On earth, almost nothing has true black-body radiation, but in space everything is close to true black-bodies.

If you look at the Planck law curves above you can see that things at room temperature (300K) would be far to the right of the visible (400-700nm). This places it in infrared, which is why we often talk of infrared as heat radiation. Like any radiation wecan see it. IR cameras can photograph things in room temperature, but this is always tricky as the camera itself gives off radiation.

As things grow hotter, their wavelengths become shorter, and eventually what is normally considered heat radiation in infrared turns into visible radiation. When things grow even hotter than the sun they begin emitting their peak in UV, and eventually the hottest things at millions of Kelvin emit X-rays from heat alone. As things grow colder they become redder to eventually be invisible to our eyes. Below is a piece of iron at my guess around 1300K, or 1000C. You can see the hottest part appear white, and as the metal grows colder it grows redder to eventually be outside thevisible spectrum entirely. Light and heat is also reflecting off the hammer above and the anvilbelow.

Spoiler: Ironworking


So this is how we can say how hot a sun is, regardless of distance, because it is the shape/colour/spectrum of the light, not only thestrength that depends on temperature. The lightfrom distant and nearby stars include an indicator of heat and total luminosity in their light.

Heat is wonderful thing and all it wants to do is give. So when a superhot sun is melting your ships, just know that all it is trying to do is share some of its warmth. And your cold metal shell is unable to give the same warmth back.

D-8: I Spy From My Little LEO

The following was originally published on the Stellaris forums by Murmeldjuret as a guest post on my series. Reproduced here with permission. Thanks, Murmeldjuret.


Today I will be continuing TheBeautifulVoid’s astroknowledge series.

In 1728 Newton first described a hypothetical cannonball that would orbit the earth like the moon did. An orbit, like the moon, but artificially constructed. In 1903 Konstantin Tsiolkovsky calculated the exact speeds and dynamics needed for artificial satellites, and proposed using liquid fuel rockets to lift them there. Since the launch of Sputnik, United States Space Surveillance Network has tracked 26000 objects in space, of which now only 8000 remain, the rest succumbing to the fiery grave of reentry. Today, there are roughly 1000 operational satellites orbiting our planet.
There are many types of satellites, the four main being:

  • Communication satellites (satellite tv, mobile phones, internet etc)
  • GPS satellite array
  • Astronomical satellites looking outward towards the stars
  • Earthward satellites looking down on earth
Spoiler: Satellite Orbits


Up until around 600km altitude, atmospheric drag is significant enough to decay orbits and eventually send the satellite into reentry. Therefore most LEO satellites are in the 700-800km altitude region. The most common orbit for earthward satellites is sun-synchronous orbit, in a semi-polar orbit. This means that they are above the same spot on earth at the same time of day. Useful if you want to take images of the ground in broad daylight, when the sunlight is the brightest and you get the best readout on your instruments.

The main method used to monitor things is namely passive remote sensing, using the light of sun reflecting off what you want to see. Our eyes are passive remote sensors. This differs from active sensing that sends out its own signal to be reflected, such as RADAR or laser inferometry. Regardless, the atmosphere between the satellite and the ground has to be filtered out. The same atmosphere that makes ground to space imaging difficult makes space to ground imaging difficult. This is however, an easier problem as we know what we are looking at. Unlike ground to space that are placed above the cloudline, what we want to see here is usually below the cloudline, and we will thus not get any data for most wavelengths.


Geographic information system (GIS) is the term for mapping the appearance, distance, and topography of the planet’s surface. All have problably seen the various map applications today. Look at google maps of a major city or university town and you can probably tilt the view to see an approximate 3D view inferred from taking multiple pictures at multiple angles. You can actually see anything that is big enough to be above noise including heights of all trees, cars, and hedges. While a more time-consuming method than just flat images, it is remarkably effective. It is even possible to show what the city would look like from the ground, even though it is taken at an altitude of above 600km. Here is the Lund Observatory as approximated in 3D shape as seen from space.

Vegetation Monitoring

Photosynthesis absorbs a specific band in the visible while green leaves reflect a specific band in near-infrared. The ratio between these forms the basis of something called vegetation index. An improved version, the Enhanced Vegetation Index, or EVI is being used more and more to determine the health of plants, farmland, forests, grass, and even plankton. See the top left of image below for Vegetation Index. Bottom left has a simple contrast false colour between absorbed visible and reflected NIR for plants. Satellites are better at telling us what is alive and what isn’t than standing 1m from it and looking at it with only our eyes.
(Image from DigitalGlobe, pretty worthwhile site if you want to see commercial satellite imaging)


With spaceborn interferometers you can measure phase shift from distance between orbit and surface. This is a remarkably exact height measure. It is used to measure glaciers, mountains, and the overall topography. Even more interesting is seeing them vary with time. How much does a glacier shrink every summer to grow back every winter, and how is the mean varying over the years?
The above image is a famous inferometer measurement of the Izmit earthquake in 1999. It measures the total change in height in a wavelike pattern from 0-2.8cm. The line in the middle is the faultline. Since any measurements from the surface would also be from an unknown height it has previously been hard to precisely measure how earthquakes change topography. But a satellite orbit cares nothing for earthquakes, and suddenly we have invariant measurements of earthquake dynamics.

Atmosphere Composition

Just like you can measure atmospheres of exoplanets with transit, you can do the exact same thing with earth. Measure our atmosphere’s spectrum by having it between a satellite and the sun/star. Since we can position a satellite with a specific altitude of atmosphere between it and the lightsource we can determine the atmospheric composition of different altitudes and locations. The ozone hole, the carbon dioxide concentrations over cities, the methane from cow farts, the water evaporation from warm sunny days, it can all be measured with both height and geographic location.

Age of Snow

One of the weirdest data I have seen is a system that measures the age of snowfall. Freshly fallen snow is loosely packed with pockets that can reflect light internally before emitting it to space, whereas older snow loses these pockets. Measuring the ratio of directly vs indirectly reflected light will tell you quite reliably how old snowfall is. Daily maps of ski-resorts telling the quality and type of snow for the resort might actually be a thing in the future.


Can you detect crime from space? Yes you can. Using vegetation index from before, you can track forestry, and thus also illegal deforestation. Both smoke plumes from burning fields and a lack of photosynthesis can be seen from space, and has been used as evidence. Big Brother is watching.


Can you see religion from space? To some degree, you actually can. With muslim attendance day being friday, jewish saturday, and christian sunday, you can map CO2 emissions over denser cities and regions over many weeks, and the statistical deviation on friday-sunday actually corresponds to religious denomination of the region and is statistically significant. (Now I don’t want a religious debate of any kind in this thread so please refrain from starting one.)

There is an apocryphal story that during the recent conflicts in Afghanistan a military satellite tracked the footprints of an ambush group back to their base. While I doubt the credibility it is actually not unthinkable. Satellites are getting to that level, and what they can tell us is surprisingly much. It is cheaper to buy an image of your town from one of several satellite companies than it is to send out a helicopter or plane to take it from the air. For those who dislike agriculture growing dependent on a more and more complicated web, how is satellite imaging as a strand in that web? 100 minerals for +1 food anyone?

D-9: The Iron Age

The following was originally published on the Stellaris forums by Murmeldjuret.


(Today we will only discuss the visible part of the universe or 4.6% of the total mass. So when I use %mass I mean % of the visible mass.)

Carl Sagan once said we are all made of stardust. There is truth in it. 90% of our mass has been forged in the fusion reactors that power stars. Though it is equally inspiring that the 10% that is hydrogen in us is the same hydrogen big bang created during the recombination 13.8 billion years ago. 10% of your mass is 13.8 billion years old. A lot of (note understatement) hydrogen and helium was created during this recombination, and it is still the same hydrogen that powers the stars.

For most of a star’s life it fuses hydrogen into helium, but as helium content grows and hydrogen content shrinks, it will eventually start fusing helium. This is usually the end of a star’s regular life as the core process that powers it and keeps it in balance falters. The results depend on startype. The new stellar reactor begins fusing helium into heavier elements. See chart below. The heaviest naturally occurring element is plutonium, though it is possible heavier elements have formed earlier in the universe but have decayed to plutonium or lighter by now.

Spoiler: Nucleosynthesis table



Above is a chart of the different types of elements in the solar system. The same ratio is about correct for the universe in general, except hydrogen andhelium are even more common. Hydrogen is about 74% by mass and helium is about 24% of the mass in the Milky Way. That leaves 2% to be shared by all the other elements. We thus say the Milky Way has a metallicity of 2%. Chemists will probably not consider carbon a metal, but since anything heavier than helium has to have been made after big bang it is a more clear distinction from an astronomical perspective.

Interestingly enough, the third most abundant element is oxygen, with a much higher concentration than any element heavier than helium. In fact, a full 1% of the galaxy is oxygen. This is why ice is not uncommon in the universe, it being a stable bond of hydrogen and oxygen, the most and third most common elements. The fourth most common element is carbon, so score one for carbon lifeforms on watery worlds being the most abundant.

Another interesting thing to note from the curve is that even numbers are much more common. This is due to helium being the main source of heavier elements and helium is two protons. There is no way to get the odd numbered elements without something to alter the regular process. Of the ten most common elements by mass, only nitrogen and hydrogen are odd numbered.

The next thing to note is the peak of iron (Fe) and nickel. Iron is a relatively heavy element, but it is still the sixth most common element by weight. 0.1% of the visible Milky Way is iron by mass. All planetary cores are iron and/or nickel based, and iron can be found everywhere. This is strange, element nr 26 beats 21 preceding elements in abundance. The reason is actually quite straightforward. Iron is the most stable nuclei in the universe. It has the lowest binding energy of all nuclei. All the lower elements give net energy when you fuse them, and all the heavier elements give net energy when you split them. There is no way to change iron without adding energy, both splitting or fusing iron costs energy. When the core of star turns to iron the process that powers a star turns to a process that takes energy from the star.

Fissile elements usually decay to lead, see its peak (Pb). The heaviest stable element lead-208 is still radioactive but with a halflife of 10^19 years. Compare that to the 10^10 years the universe has existed. Everything heavier than iron might be radioactive but with halflives unimaginably long.

The hydrogen created at big bang is limited, and it is being burnt in every star, in every galaxy, in every cluster, every second. For each atom of iron it creates, there is less fuel left to power the universe. If nothing else ends the universe as we know it, the hydrogen and helium will run out, and all that would be left is a universe sized cloud ofiron glittering from the light of long gone stars.

D-15: Large Adjective Astronomy

The following is another guest post originally written by Murmeldjuret on the Stellaris forums. Reproduced here with permission. Thanks, Murmedjuret.


As much as we all love Hubble Space Telescope and its images, it is getting rather old. 26 years old as a matter of fact. There has been talk about sending up a newer and larger space telescope, but the cost would be well, astronomical. Why space telescopes then? Well because they do not have a pesky atmosphere between them and space. The atmosphere is turbulent, varying in heat, pressure, and humidity. The atmosphere also heavily absorbs in some wavelengths, effectively making them dark to anyone on the ground. Notably ozone in ultraviolet and water in infrared. Seeing in infrared is very important as dustclouds and nebula more easily block shorter wavelengths. Many stars and galaxies are invisible behind dustclouds unless you look at them infrared.

The successor to Hubble Space Telescope (the James Webb Space Telescope or JWST) is planned to be launched in 2018, and it is primarily a near infrared (NIR) telescope, capable of seeing from 600nm (visible orange) to 28000nm.
It will be better than anything we now have at imaging brown dwarfs, planets, kuiper belt, comets, galactic cores, and anything else cold or hidden behind a dustveil. It won’t be in the same RGB colours we see, but it will still generate detailed images in false colour.

Problem with infrared is that anything cold will also be seen by it, namely the interplanetary dust in the solar system. The sun and dust could also warm it so much it takes pictures of its own lens, welcome to infrared optics. It has to be launched to L2 lagrange point, 1.5 million kilometers behind earth, so the dust around earth is far away. It has a massive sunshield to block the sunlight and cool it down so it doesn’t see its own optics.

Its mirror’s effective diameter is 6.5m, which compared to Hubble’s 2.4m is quite a bit larger. It is actually too large to fit on any launch system and has to be folded up to fit inside. It will be in a elliptic orbit at L2 lagrange point, following earth but orbiting the sun and not earth. It will use a few m/s delta-v per year to maintain the orbit, so the L2 orbit is not exactly cheap.

Spoiler: James Webb Space Telescope


Now, what is the alternative to massively expensive space telescopes? Well that is ground based telescopes, currently the highest resolution is 0.001 arcesconds from the Very Large Telescope Array, with its four 8.2m telescopes. With the advent of high energy lasers, ground based telescopes have gotten a lot better. 0.001 arcseconds resolution is about 1 pixel being 2x2m on the surface of the moon. The only way before to counteract the turbulent atmosphere was to find a stationary star and using the stars distorted movement to compensate with adaptive optics. This is all fine and well, but most of space does not have a stationary star. The modern solution is to use a high energy laser at a specific wavelength that makes sodium fluoresce, or glow.
In the upper atmosphere there is a sodium layer, and this high energy laser creates an artificial guidestar in that layer. We can use this artificial guidestar to compensate for the turbulence.

Though a 8.2 meter telescopes on earth don’t even come close to the resolution you get from 6.5m in space. What does? 39 meter telescopes. The greenlighted 39m European Extremely Large Telescope, yes that is the name, is planned to be finished in 2024, and it will have a resolution more than 10x what Hubble has. NASA also has a 30m telescope under construction, but it looks like it might get cancelled from budget, religious, and a 39m telescope already being underway. Like JWST, ELT will take images of distant solar systems and galaxies, meaning we can resolve planets, not just infer them from looking at the star.

Spoiler: VLT with guidestar laser

Spoiler: ELT Dome with car for scale


So JSWT will be able to take the best images we have ever seen before of space, and it will launch in two years, and a few years after that we will get a telescope we don’t even know how good it will be. JWST will be able to see things we have never seen before from its high resolution infrared system, and then we will get even higher resolution images from ELT. If you want to compare cost, the JWST lands at 8.8 $billion, and E-ELT at 1.5 $billion.

Spoiler: Comparison between mirrors


Here is comparison of the different space imaging systems, with a human, tenniscourt, basketball court, and a Radio Telescope for comparison. JWST and Hubble are down in the bottom left, E-ELT is on the right. There is a human just below E-ELT for scale. The big grey ring in the background? That is the other proposal that was discarded in favour of ELT. The European Overwhelmingly Large Telescope with a 120m mirror. Yes, that was the planned name. It was cancelled because we don’t even know if it is possible to build a dome to hold it. The ELT is already a composite mirror of nearly 800 panels, and it would only be a fraction of the requirement for OLT.

So what can we expect?
Well the first mission of JWST is to look at KIC 8462852, which we have been unable to rule out contains intelligent life. High resolution infrared images would tell us a lot about the system.
This is an image of Formalhaut b, a massive planet around an A-sequence star we saw two episodes ago. We can already resolve it to several pixels. This is what we might see of it with JWST and ELT:

Spoiler: Disclaimer: I photshoped the right image, mostly based on guesswork


The above image might not seem like it but we see both a moon and rings surrounding the planet, scattered dustcloud and noise (it is scaled up from 60×60).

As wondrous as Hubble’s images have been, they are about to look very outdated.

D-22: That’s No Star

Another guest post from Murmeldjuret, republished with permission.


If you look near the constallation of Virgo, you can see this thing:

Spoiler: Hubble Space Telescope is Great


But wait, that’s no star…

That is the M87 galaxy. Yes, the bright yellow orb in the upper left is a galaxy, equal in diameter toour galaxy. Despite being about the same diameter as the Milky Way, M87 weighs a few thousandtimes as much, as it is practically spherical, whereas the Milky Way is a disk. M87 is a supergiant elliptical galaxy in the Virgo GalacticCluster, part of the Virgo Supercluster, the same Supercluster a certain Milky Way resides in. Here is picture of the Virgo Supercluster:

Spoiler: Virgo Supercluster


M87 is near the centre towards us in the Virgo Cluster. In fact, that is rather close, at only 53million light years away. That is just 20x the distance to Andromeda. Our closest galaxy.

Here is an image to scale, with each pixel equal to 50kpc, or 163000 lightyears, or 1 550 000 000 000000 000 meters, or 1.55 billion billion meters. Per pixel.
Yes, that blue line is longer than the distance between us and Andromeda. That line is not your friend. That line is a jet of ejected electrons and a continuous Gamma Ray Burst. Yes, a continuous stream, the size of a galaxy, of the stuff we saw back in Luck.┬áThe electrons? We don’t know that much actually. The instruments tell us that they move faster than the speed of light, so we assume the instruments are wrong. Their total energy is somewhere on the order of 10^50 Joules. Or equivalent to the energy output of the Milky Way, for the past three hundred thousand years.

Had it been pointing at us the visible part would cover more than 1/10 the distance between the galaxies. Exactly how bright it would shine is hard to say, but it would be clearly visible from every part of the galaxy. How strong the radiation would be is also hard to say, but it is one of thosequestions that we are glad we don’t have to answer.

Now what kind of monster can create that thing. Well the supermassive black hole is more 3.5 000 000 000 times the mass of our sun, 3.5 BILLION times the mass of our sun. Or more than 1/500 of the mass of the Milky Way.
It is one of the biggest objects we have everdetected.

The black hole appears to not lie in the galactic centre, which is weird. There are two theories, equally scary awesome.

  1. This is due to relativity and sensible physics breaking down due to the mass andrelativistic jet of matter. It actually lies in the centre, but the laws of physics get bent so we see an illusion.
  2. The Black hole only has one jet, which it shouldn’t, but if so that jet is accelerating the black hole out of the galaxy. It would explain why we only see one beam, but the missing other beam can be explained by A.

Life as we know it can not evolve in M87. The extreme activity going on bathes the galaxy in X-Ray clouds. In fact, the X-Ray cloud has pressure waves and storms, much like a weather system. Lead jacket is minimum packing if you even want to get close to M87.

Despite being many times the mass of the Milky Way, M87 actually has less of a dust cloud by a large margin. Either it has been blown away by the jet or been swallowed by the black hole.
M87 does have a very large number of Globular Starclusters, small groups of stars surrounding a galaxy. About 12000 of them, whereas the Milky Way only has a few hundred. One of these globular clusters is called HVGC-1, or High Velocity Globular Cluster 1. It has been ejected from the galaxy at a speed of several thousand kilometers per second. In fact, it is going so fast it will one day leave the Virgo Supercluster. It got fed up with the Virgo family and is departing for a distant void. Simulations indicate this is due to there being, or having been, two black holes in the centre of M87. It is very likely M87 has previously swallowed other galaxies.

According to some of the end of the universe ideas, it will one day swallow all of the Virgo Supercluster, Earth included.
After all, it isn’t very far, and we are movingtowards each other.

D-23: The Hunt for Exoplanets

The following is the first guest post on the series, written by a person called Murmeldjuret on the Stellaris forums. It has been reproduced here with their permission.


Today, by request it will be on how to detect exoplanets. I have made it rather exhaustive, so get your coffee ready.
For those of you wanting to learn of the things that can and will want to kill you, there will be moredeadly friendly lightshows tomorrow.

The main problem with detecting planets is that they are very small compared to a star. They don’t give off light, and not being on fire tends to leave them cold. In fact, we have only directly seen a very small set of exoplanets. Wikipedia lists 20, all of them larger than jupiter. Nasa lists 33. All the other thousands of planets we have “detected” we have actually never seen. They have all been inferred from observing the light of the star they orbit. Inferred is mathspeak for qualified guessing, meaning the changes in starlight is most likely from a planet, but we are not 100% sure. Some of them we are really certain about, some like the previously described Gliese581d are less certain and are considered disproven one year to be proven the next.

There are two primary methods to finding planets in other solar systems. Planetary transit and radial velocity also known as doppler shift. Both require the planet to rotate over or nearly over the star, meaning we can only detect planets around stars whose planetary disk aligns with the star and Earth. We will not be able to detect planets from the majority of stars out there. If a star has not yet been proven to have a planet it might still have one, only we are unable to detect it. I should point out that there are methods to detect planets by two other means but we have seen only a dozen this way.

Planetary Transit:

If the planetary disk intersects a star, for parts of that planet’s year it will lie between us and the star. Much like a solar eclipse from our moon blocks sunlight, only much fainter. By measuring the light from the star over a long time we can see how it varies, and if it has any periodic and regular variation. An ideal planet transition is shown in the diagram below, with one period being a year for the detected planet, the y-axis is total luminosity from the star:

  • A: Planet is obscured by star, only the star is visible. Useful as reference point.
  • B: Planet is reflecting sunlight, we could judge albedo (reflectiveness) of planet if we trust the size and orbital distance of the planet. This is useful since the albedo of different wavelengths is dependent on absorption and absorption is dependent on what elements it is composed of.
  • C: Planet is starting to block sunlight, and depending on how sudden the drop in sunlight is we can see if there is an atmosphere. Given enough spectral and spatial resolution we could determine what elements the atmosphere has, especially watervapor. With a sufficiently precise spectral resolution we can also see if it has molecular oxygen (O2) and carbon dioxide. Right now this is only viable for gas giants, not so for rocky planets.
  • D: Planet is obscuring sunlight by a very fine amount. We can also estimate how much heat the planet captures which is a strong indicator of how much atmosphere it has.

All of these combined with total rotation time and solar mass/energy gives us a equation system that can reasonably well give us size, mass, and orbit of planet. This is how we today say how large it is, how far from the star it is, and if it is in the habitable zone. We can also determine if it has an atmosphere and how much atmosphere it has when we get more detailed equipment. With even better equipment we can also detect oxygen and carbon dioxide, and therefore if it has breathable atmosphere and if carbon material for life exists. All without ever seeing the planet.

Radial Doppler:

Most of you have probably heard of the doppler effect. It is when an object moving towards you increases in frequency (gets blueshifted), and what makes things moving away from you get redshifted. It is why police sirens zooming past sound high pitched while approaching and low pitched after they passed. The same is true of light, and since almost everything distant in the universe is redshifted the universe must be expanding.

Another physical phenomena is that two objects in orbit always rotate around a common centre of gravity. It is very noticeable for binary starsystems, less so for Sun and Earth. But this means that every star with a planet orbits a commoncentre of gravity that is not quite the centre of the star. Therefore every star spins in a very small orbit, barely noticeable from just looking at it.

In the event that the planet is on a disk close to being aligned with us and its star, then we can use radial doppler to see the planet(s). This means that the star is at two points going perpendicular to us (not towards us or away from us) and at one point it is moving towards us, and at another moving away from us. This will in turn by a very small degree dopplershift the wavelength of the star. The star and its spectra are wobbling from the planets gravity on the star. This is periodic with the year of the planet and scales with the relative mass of the planet and its star. Since we can know the mass of the star we can also estimate the orbit and mass of the planet. This can unlike the transition method detect planets that are not transitioning directly over the star. Though we will not find the exact mass unless we can compensate by using other means. The European HARPS installation can determine the speed of a stars rotation within an error of 1m/s.

The biggest drawback of both systems is that both are dependent on the relative mass and size of the planet vs the star.

  • Detecting planets around red dwarves is easy, detecting planets around blue supergiants is nearly impossible.
  • Detecting small planets the size of Earth is very hard, detecting gas giants is easier.
  • Detecting planets far away from the star is very hard, detecting a gas giant in close orbit is easy.

See the chart below of EU’s database to see the mass of detected planets, in jupiter masses. And another below with both mass and distance to their star.

So what does this mean?
We have found over two thousand planets so far. Almost all of them in the past two years. Averagingat almost 3 planets per day. We will detect about 70 new exoplanets from today to release.

This has challenged previous assumptions of planet frequency. Previously, planets were thought to only orbit main sequence stars, were uncommon, and multiplanet starsystem are exotic. We have had to revise our understanding of the universe. The estimation is now that there is on average more than 1 planet per star. Meaning that theMilky Way contains something on the order of 100-400 billion planets, probably more.
The local Virgo Supercluster might contain tens of thousands of trillions of planets. (I just took a reasonable number, do not quote me.) If only one planet in a trillion 1/1000000000000 contains life, there are still over ten thousand life bearing planets in our local supercluster. And our local supercluster is one of many millions of detected superclusters.

If the chance that a planet is inhabitable is 1 in a billion, the Milky Way contains 100-400+ habitable planets, reminescent in order of magnitude to a certain spacegame…

In short, there be planets out there.