Category Archives: FAQs

So you think you’ve found an exoplanet…

One of my day-to-day jobs as an exoplanet astronomer is to look through transiting exoplanet candidates and find those that are actually from exoplanets. However, scrolling through Kepler, K2 or TESS lightcurves is something anyone can do – professionals and amateurs alike. Therefore, it may be useful for me to go through a few of the steps we use as professionals to determine what makes, and what kills, a good planet candidate.

First some key points:

  • The majority of “dips” found by photometric surveys are not planets – they are false positives like eclipsing binaries (stars crossing stars), or even just junk noise from the detector.
  • Even good-looking planet candidates are not, on their own, interesting or publishable. To confirm a planet there needs to be a lot of follow-up time on ground-based telescopes, as well as a lot of complex data analysis.
  • But you can still make a contribution to exoplanet science by finding previously-missed planet candidates and passing them to astronomers.

So let’s get started…

1. What makes a transiting planet candidate?

Planets can be found by searching the shadow they cast as they pass in front of their parent star – effectively blocking a bit of starlight for a few hours. This is the famous “transit” effect. However, we don’t see the shadow directly (except during transits within our solar system – i.e. the transit of Venus which is shown below). Instead, the star is seen as a “point source” with all the light from the star smeared onto a few pixels on a detector. But, by adding up the light from the star across those pixels and plotting it against time (this is “photometry” and produces a so-called “light curve”) we can see the “transit” in the data – a U-shaped dip. We can immediately throw out signals that jump up (instead of down), or asymmetric events which drop down without ever jumping back up again – planets only ever cause symmetrical dips.

The 2012 transit of Venus and associated transit light curve (from JR Davenport)

Planets are also strictly periodic – their signals repeat precisely once every orbit. Therefore, good planet candidates should have repeating dips as is shown below. However, this isn’t strictly necessary – long-period planets may transit only once. But we can still assess these monotransits with the same rules.

A periodic planet seen in TESS

This strictly periodic nature means we typically search do not find planets by looking at lightcurves – instead we search through a range of potential periods and stack data together (e.g. using BLS or TLS). If the period matches the real period, each transit stacks together, and their combined signal is strong, producing a spike on a periodogram. This lets us delve into the noise and find shallower transits.

The phase-folded photometry and periodogram of MASCARA-4b

2. Information from the transit

So you have found a dip… The most useful information now comes from the depth of the dip. The reason for this is obvious when you think about the geometry of a transit – the bigger the transiting object is, the more light it blocks.

Using geometry, the area of the star emitting light is πRs2, while the area of that star being blocked by the planet is πRp2, so the depth is the ratio of these, i.e. (Rp/Rs)2.

Transit depth is influenced by both planet radius and stellar radius (from A. Vanderburg)

Planets are, by definition, objects smaller than stars. Typically, giant planets top out at about 20 earth radii, or about 0.2 solar radii. So, for a star like the Sun, the maximum possible depth from a planet transit is 0.22 = 0.04. So if you found an interesting dip which seems to drop by greater than 4%, then it’s not a planet – it’s one star orbiting another, or what we call an eclipsing binary (or EB). And even dips between 1% and 4% are far more likely due to EBs than planets.

Not every star is like the Sun. But thanks to Gaia there are estimates of stellar radii for almost every star in the sky. If you’re looking at TESS data you can check the TESS ID in ExoFop-TESS, while MAST has data for K2/EPIC IDs and for Kepler IDs. Now you have a transit depth and a stellar radius you can estimate a planetary radius (Rp= Rsdepth).

Giant stars 10 times the radius of the Sun will effectively never have detectable transiting planets around as the limit drops from (0.2/1)2=4% to (0.2/10)2=0.04%. Smaller M-dwarf stars could host planet that are almost as large as their stars themselves, however giant planets are extremely rare around M-dwarfs, and typically the <4% rule holds for all stars smaller than the Sun too.

3. Transit Shape

It’s not only the depth of the lightcurve which contains information about the transiting object – the shape of the transit does too. Typically, us astronomers would model the lightcurve with a transit model, and this would produce parameters including a better estimate of planetary radius, period, as well as all the transit geometry. For this modelling, programs like Jason Eastmans’s exofast2 are good places to start.

However, for just a simple sense of what else the transit tells us, here’s a couple of points:

  • V-shaped dips are caused by objects that do not fully traverse the stellar disc. This means their radius is larger than the radius calculated using (Rp/Rs)2. The exact boundary depends on many parameter, but even a 1%-deep V-shaped dip (i.e. one that does not have a flat bottom) is far more likely to be a binary than an exoplanet.
For the same sized object, a grazing eclipse (turquoise) hides the true depth & size.
  • Transit duration (tdur) should match the stellar density (ρs). There is a bit of weird geometry to do before it’s obvious why this is the case, but transit duration is proportional to the stellar density & planet period where P/tdur3 ~ 5000 ρs. So for a P=10d transiting planet with a 0.1-day transit duration (2.4hrs), the computed density roughly suggests a star slightly denser than the sun (with ρs~2). If this is hugely discrepant with the actual density of the star (as provided on ExoFop or MAST), it suggests the object is in fact an EB. However, the transit duration can vary by a factor of 2-3 here, so only really large discrepancies (a factor of >10) are useful.

4. Out-of-transit features

Transiting planets can be thought of as simple black spheres which are only seen when they cross the star and block it’s light. Eclipsing binaries, however, are due to stars of their own and emit their own light, and therefore we often see their effects out of transit as well. So if there are any of the following features in the lightcurve, we can safely call this candidate an eclipsing binary and not a planet:

Secondary Eclipses

While planets cause one dip per year, eclipsing binaries often cause two – one when the dimmer star passes in front of the brighter star (the “primary eclipse”), and another smaller dip when the smaller star passes behind it’s larger host (the “secondary eclipse”). This might look like one of two things:

1. A smaller dip seen consistently at the same place between each “transit”:

2. A slight difference in depth between odd transits and even transits (caused by two similar-sized stars):

For circular orbits, the secondary eclipse should happen when a star is half way round its orbit (i.e. at phase 0.5). But this is not always the case – eccentric orbits mean that often that the position of the secondary eclipse is offset from this mid-point.

Synchronised sinusoidal variations

Stars are typically variable due to starspots & other effects. However, for planets, those up-and-down wiggles are never linked to the planet’s period. If the wiggles in the lightcurve are synchronised with the transit period, then that’s a sure sign that you’re looking at an eclipsing binary.

That’s because, when the transiting object is massive enough to influence the primary star and bright enough to produce its own light, they can produce sinusoidal signals linked to the binary period due to:

  • tidal effect of the smaller star synchronising the stellar rotation of the primary star so starspots spin with the companion;
  • a hot-spot located at the noon position of the smaller star;
  • the gravitational pull of each star on the other stretching the stars into elongated ovals creating two cycles of “ellipsoidal” variation per period;
  • the velocity of the smaller star’s orbit causing a “beaming” effect where light is red- & blue-shifted into & out-of the colour range that the spacecraft observes;
  • some combination of all of these effects.
Synchronised ellipsoidal variation at the candidate period
Out-of-transit variability at the candidate period

5. Extra Checks

There are a couple of final checks which require a little more effort than merely looking at the lightcurve.

One of the key things to check is: is this already a known candidate? For TESS & K2, this should be obvious on the ExoFop page for that target. For Kepler you can check if it’s on the Kepler candidate list. In both cases, these catalogues should also contain a disposition – i.e. whether the Kepler or TESS vetters have also determined it to be a planet (PC), or a false positive (FP, NTP, EB, etc). If it has already been detected (and at a period and epoch that matches), there is not much that you can do – astronomers already have this on their lists and it will be (or will have been already) published without your help. That’s the way it goes sometimes!

If it’s not known, then there’s a chance you have an interesting target that astronomers missed!

One of the key points is that the dip should be occurring on the brighter target star in the field you’re looking at. If a star which only contributes a small fraction of the light is causing the dip, it must be dimming proportionately by a large amount. For exmple, if you see a 1% dip but it’s actually coming from a nearby star which only provides 4% of the light, then that star must have dimmed by 25% – far too deeply to be a planet! The best way to do this check is to:

  1. Check the lightcurves of nearby stars to see if they are causing the dip.
  2. Check the “centroid position”. The centroid is a weighted average of the flux coming from the star onto the detector, and any shift in this between the in-transit images & out-of-transit images moves is a sign that a nearby unresolved star is causing the dip and not the target star.
  3. See if the depth of the transit increases as a function of the “aperture” size (i.e. the number of pixels used to compute the lightcurve). A larger depth at larger apertures suggests the star is a blended nearby star not at the position of the target star.

Another useful check is using Gaia’s radial velocity and astrometry error – if these are high it’s typically because the target star is in fact two stars which are constantly in motion. This can be done by typing the coordinates into a Gaia cone search and scrolling down to the astrometric_excess_noise_sig and dr2_radial_velocity_error parameters. If either of these are higher than about 5, it’s a sign that the object is a binary (though this is not always true).

6. What Now?

So you’ve found a candidate which seems to pass most of these tests… What now?

As I mentioned at the start, it isn’t just a case of “upload it to the exoplanet database and move onto the next one” – these databases only contain planets that have been published in the academic literature, and have been independently & statistically verified.

So, if you’re simply an amateur scrolling through lightcurves for fun, what do you do now? One nice next step would be to share it on the planethunters message boards as the interesting cases will make their way to astronomers. Alternatively you can directly email the candidate to an exoplanet astronomer (like me), and we’ll let you know if we think the candidate is interesting, and likely pass it on to someone better suited to studying (and eventually publishing) that candidate as a planet. But – fair warning – even if you think your candidate is awesome, and even if an astronomer agrees that it’s likely a planet, that doesn’t necessary mean that it’s easy or interesting enough to get published. Some cases are very tricky and require a lot of extra observations – something that typically only happens for interesting cases.

Another avenue, only to be taken for candidate you are sure are real, is to propose it as a Community TOI through ExoFop-TESS. However, it’s best to get confirmation from an exoplanet astronomer and TESS member before doing this.

I hope this proves a useful guide! Happy hunting.


For more basic info on transiting exoplanets, check out Andrew Vanderburg’s light curve tutorial and Paul Wilson’s exoplanet transit method page.

Frequently Asked Questions.

For anyone wondering, in the most basic terms, what it means to find new planets and how we are able to do it, here’s a short blog I put together based on what people seem to want to know…

So what do you do?

I use telescopes to hunt for planets around other stars.

How does that work? Do you just find new dots of light in the sky?

Kepler-62f_with_62e_as_Morning_StarStars are really bright. The Sun, for instance, shines tens of billions of times brighter than any of the planets. Not only that, but the stars we see in the night sky are also incredibly far away. Imagine holding the Earth and the Sun within your palm. (By the way, the Earth in this scenario is the size of a human red blood cell). On that scale, Mars is just beyond your wrist and Saturn is by your elbow. The nearest star, however, is still more than 7km away! And what we want to do is spot a fleck of dust around that. It is as impossible as it sounds – even with the best telescope on Earth, we can’t spot planets directly in this way.

So how do you know if a planet is there?

While we may not be able to spot the planet’s light directly, there are hints in the starlight itself.


Everything that has mass has a gravitational pull; and while most people know that the mass of the Sun keeps the Earth in orbit around it, few people know that the Sun is also in motion around the Earth. That orbit is a tiny circle: only 450km in radius compared to the 150million km orbit the Earth follows once a year. Larger planets like Jupiter cause correspondingly larger effects. So, by detecting the motion of a star, we can figure out how big a planet is, and how far away from the star it must be.

This was how the first planets were found 20 years ago; by splitting starlight into its colours and using tiny shifts in the ‘barcode’ of elements to determine the motion of the star. It’s also how the new Gaia satellite will hopefully find tens of thousands of new planets, by precisely measuring their locations and spotting these tiny wobbles.


Another method is to hunt for the rare occasions when planets pass in front of their stars, like the moon during an eclipse. However, even the largest planets only block less than 1% of the starlight during these transits. So to find planets this way we not only need telescopes that can look at thousands of stars at a time, they also need to be really accurate at measuring the star’s brightness. This is something we have been able to do from the ground, with surveys like WASP and HAT, and from space with telescopes like Kepler.

So which method do you use?

I hunt for planets using the transit method. At the moment I am trying to find long-distance Jupiter-sized planets using the WASP survey. This uses 16 relatively small cameras in South Africa and Chile to follow the brightness of millions of stars. So far we’ve found over 100 planets this way, with dozens more in the pipeline.

How many have you personally found?

Finding planets really is a team game. I have helped out at every stage from identifying the first signs of planet transits, to following up transits with better telescopes to check they’re real. So, depending on how you count, somewhere between zero and half a dozen.

Do you get to name them?

Nope :(. Any planets we find get named after the telescopes, eg WASP-134b. Not exactly the most captivating names, I know!

Have we found any planets like Earth?

EarthMoonSmaller planets, especially ones far away from their star, are a lot more difficult to detect. To detect such planets using the radial motion of a star requires measuring the star’s velocity to better than 10cm/s. That’s slower than your average tortoise! Remarkably, there are some instruments being built that could do it.

As for the transit method, Earths crossing their star dim the light about 0.008%. That’s about the same as watching for a piece of dust crossing in front of a lightbulb. But, remarkably, we can do it! The Kepler space telescope was able to monitor the brightness of more than 100,000 stars down to a few parts per million. And it found dozens of Earth-sized worlds that look very similar to our own.

Will there be Aliens on them?

Maybe. Thanks to missions like Kepler, we now know that there are hundreds of billions of planets like the Earth in our galaxy. And we know that life sprung into existence on at least one of them. So there’s no reason to think it wouldn’t have done so elsewhere else too.

Whether intelligent life is common is another question – we’ve been looking for decades and found absolutely no evidence. My own feeling is that, while simple microbes might be common in the universe, technologically advanced life probably isn’t. It took 4 billion years and a handful of chance events for intelligence to evolve at all. And the way our species is going, surviving even for a few thousand years more (~0.0001% the lifetime of the planet) seems unlikely.

Please feel free to ask any more questions in the comments! I will add some if and when I think of them.

What’s In A Name?

Hundreds of astronomers across the globe are currently searching nearby stars for a fleeting glimpse of astronomical gold dust: exoplanets. I am also part of the search, scanning through terabytes of data taken by the WASP and NGTS telescopes looking for the distinctive signal of a distant world crossing its star. Thanks to the mountains of data from NASA’s Kepler probe, it is now even possible for amateurs to go online and help out. And thousands of people have taken part, spurred on by the chance to become the first person in history to lay eyes on a new part of the universe.

It is a thrilling quest, but the question on everyone’s lips is this: do you get to name it? Surprisingly enough, the answer is a ‘No’. Or maybe a ‘Not yet’…


Here on Earth it has long been custom that, for whatever it may be, the discoverer becomes the namer. Columbus, Cook and Magellan all took pleasure in naming new lands, doctors such as Alzheimer or Asperger gave their names to their respective disorders, even some recently named animal species include Attenborosaurus conybeari and Heteropoda davidbowie in honour of the researcher’s heros. Chemists discovering new elements are given a relative freedom over naming their discoveries. Even in astronomy, comets are named after their discoverer with names such as Lovejoy or McNaught often gracing comet codes. Exoplanets, on the other hand, are a very different kettle of fish.

The problem with naming planets comes from the stars they circle. As nice as it would be to name every object something eye-catching like ‘Permadeath’ or ‘Baallderaan’, to avoid confusion the name of the star must be listed first. This is much like the way biological names come with both genus (Homo) and species (sapiens). So how do we end up with names like HD80606b whereas biologists get Bushiella beatlesi? The first part comes down to how we name stars.

Too Many stars to Count

Unlike islands or animals, there exist a near infinite plethora of stars. Our galaxy alone has more than 100 billion. Attempt to name each in the Linnaean style and you would quickly run out of words (and sanity). Early sky-watchers soon realised this and, after giving a few hundred stars colloquial names such as Vega or Pollux, settled for simply numbering the stars by brightness in a certain area. This ‘Bayer’ designation, cooked up in 1603, ranked the stars from alpha down to omega and beyond. For example the brightest in the Centaurus constellation is Alpha Centauri, our Sun’s nearest neighbour. With limited telescopic power and Greek and Latin characters, Bayer gave up after about 1500 stars.

More recent surveys have used telescopes to attempt to sweep the rest of the sky into some sort of order. This has resolutely failed, with the majority of stars having numerous names under many different catalogues (HD, HR, Gliese, or HIP to name but a few). Each of these official catalogues simply orders the stars by number, giving rise to the cumbersome alphanumeric system we see today. {NB: Despite what some might insist, naming a star has never been done via gift subscription companies}. So, thanks to the sheer number of star systems, the sky is a mess and there would seem little hope of sorting it out. 

GJ581 Planets

But forgetting the star for a second, once a planet is found we do get to add a ‘species name’ to the stars, right? Dont get your hopes up: this is normally the lower-case letter b. The lower case shows it to be a planet (as opposed to ‘B’ which would designate another star) and the ‘b’ designates it as the second object in the system after the star itself. In multi-planet systems things get even more confusing, with the order of names increasing not outwards from the star but simply in order of which was discovered first. For example GJ581e circles within the orbit of ‘b’ and GJ 581g is sandwiched between ‘c’ and ‘d’. However, this fundamentally makes sense: planets in the same solar system are given names reflecting their sibling nature.

It may be a dysfunctional system that results in far-from eye-catching names, but it is one at least partly grounded in reason. The alternative, of letting discoverers name the planet whatever they want (my personal choice would be Hughtopia), would ultimately end in confusion and a lot more angry shouting matches at conferences.

Even worse, a whole host of recent crowd-sourced websites have sprung up attempting to get the general public to name the 100-strong list of current exoplanets (for money, of course).  The International Astronomical Union (IAU), who ultimately decide on the names of everything in space, have even given support to public-generated naming systems. The feeling among astronomers, though, is that such a move might not be such a good idea.

But is there a middle way? Could the ordered nomenclature remain intact while giving at least some naming rights to the discoverers? The Planetary Habitability Laboratory recently proposed a system that would retain the star name but allow free reign over the planetary name, for example allowing Alpha Centauri B b to become Alcen-B Rakhat. It is an intriguing idea, and one that could help improve the public perception of astronomy. I, for one, am still hopeful that ‘Betelgeuse Hughtopia’ can become a reality.

[Relevant XKCD:]