My Current Research into PCEB Exoplanets

 Exoplanets, General Astronomy, Post Common Envelope Binaries  Comments Off on My Current Research into PCEB Exoplanets
Aug 172021

It’s about time I got my astronomy blog going again so I thought I’d start by explaining what I’ve been up to over the past two years or so in an area of study on the possible existence of exoplanets around a certain type of binary star system known as Post Common Envelope Binary (PCEB) systems.

I got into this area of research after meeting fellow Selsey astronomer John Mallett in 2019. John is part of a small group of amateur astronomers who have been researching in this area for a few years and I was recruited into their number almost straight away. The group goes by the name of ALTAIR Eclipsing Binary Research Group and here is a link to the about page of our website which is written and maintained by myself. The site is private to the four of us who are currently members, but the about page is publicly accessible. You can access all the published papers from the ALTAIR group on the About page – I have only been involved in the most recent one, but I am very busy writing parts of a new paper which is in progress.

The technique used to determine if an unseen third body (an exoplanet or perhaps a brown dwarf) might be orbiting one, or both of the stars in the PCEB system is called the method of Eclipse Time Variation (ETV). The rest of this article explains this in more detail.

Let’s clear up some terminology…


Exoplanets are planets that orbit stars other than the Sun. It is an amazing fact that we had no evidence of the existence of any exoplanets until 1992, although it was widely assumed that the Sun could not be unique in having a family of planets (and other smaller bodies) in orbit around it. Such are the vast distances between the stars, it is only in these past few decades that the technology has existed to detect the existence of exoplanets, and it is only very recently that any exoplanets have been directly observed – most are detected by more subtle, indirect means – more about this shortly as it is pertinent to this story. To put it into perspective, there are now well over 4000 ‘confirmed’ exoplanets, and the number has been doubling every 27 months. However, only 16 exoplanets have been discovered using the ETV method that we, the Altair Group, are studying.

See NASA’s exoplanet website for lots of great information.

Post Common Envelope Binaries (PCEBs)

Binary stars are extremely common in the universe. A binary star system consists of two stars in a common orbit around each other. Star systems with three or more stars also exist. The specific type of binary systems that we are interested in are known as PCEB systems and evolve in a particular way, which I will briefly describe.

It is likely, in a binary system, that one of the stars will be more massive than the other. The more massive the star, the faster it ‘burns’ through the various stages of stellar fusion, and the faster it evolves. In this way, the more massive ‘primary’ star in the pair more quickly evolves into its red giant phase and expands enormously. (Our Sun will become a red giant in about 5 billion years). The primary star expands to the point where matter in its outer layers starts to overflow and transfers to the smaller secondary star. Thus, a common envelope of material is formed between the two stars. The secondary, typically a main-sequence star at this point (like our Sun), cannot cope with all this material that is being donated to it by the primary partner and, eventually, the common envelope is ejected from the star system altogether. The key here is that the departing envelope material takes angular momentum away with it, and the law of the conservation of angular momentum dictates that the two stars must be left with less angular momentum. This they achieve by moving closer together, which also causes their orbital period to decrease to the point where they spin around their common centre of gravity in a stunning, but typical time of two or three hours! During this process, the primary star will evolve on and will become either a white or red dwarf, whilst the secondary star stays as a main sequence star for a long period due to the fact that it started life as a fairly low mass star.


First, I should mention that it is generally not possible for us to make out the two separate stars using a telescope from here on Earth – certainly not with an amateur telescope, and this means we just see a single resulting point of light. So where does the eclipsing bit come in?

If the stars are aligned, even roughly, to our line of sight such that during their orbital dance, the stars move in front of each other, we will notice the following effects:

  • When the stars are side by side, the combined point of light that we see will be at its brightest
  • When the fainter of the stars moves in front of the brighter, we will see a larger dip in brightness (the primary eclipse)
  • When the brighter of the stars moves in front of the fainter, we will see a smaller dip in brightness (the secondary eclipse)

These effects are clearly illustrated in the graph of brightness (magnitude) over time shown below. We call this a light curve. I made these measurements at my observatory here in Ham of brightness (a process called photometry) of a star called HS0705+6700 (aka V0470 Camelopardalis) over a 4.5 hour period. The big dips are the primary ‘minima’ and the small ones are the secondaries. The binary orbital period of this star system is about 2.3 hours. The strange time scale at the bottom is in Julian Days with a correction for differences in the Earth’s position with respect to the barycentre of the Solar System. See here for more details about BJD, but know that this is the standard way astronomers in this field represent the date and time of the measurements. You can see on the graph that the primary dips are separated by just under 0.1 of a day which is where the 2.3 hours comes from.

Rotating Like Clockwork?

We might expect that the resulting whirling pair of stars will eclipse each other repeatedly and accurately, like celestial clockwork, and this would be true if we just considered the Keplerian equations of celestial mechanics. It would mean that, if we can measure the time of the minima with sufficient accuracy, we can work out the binary period and therefore we should be able to predict the time of any eclipse into the future (or back into the distant past).

This does actually hold accurately to a certain degree. We do indeed predict when eclipses should occur so that we can observe and measure them over the months and years after we have established the exact date and time of the first observed eclipse, and we are able to do this because, although the period is not exactly following Keplerian clockwork, it does so to a very high degree of accuracy – easily enough to pin down the time so that we can go out on the predicted night and be sure of capturing the minimum dip so that we can re-measure and refine the date and time once more.

However, in the real world there are several other known physical effects that can very slightly change the orbital period of the two stars. I will go into this shortly, but first let me introduce the tool we use to highlight any differences that might be seen in the binary period over time. The graph is called the O – C (pronounced “O minus C”) which stands for the Observed minus Calculated (or Computed) graph. An example taken from within our ALTAIR website for our friend HS0705+6700 is shown below:

What we see in the O – C chart above is the Cycle or Epoch number of the eclipse plotted on the horizontal x-axis. The number of seconds difference from the expected calculated date and time of each minimum point is plotted on the y-axis. The different colours illustrate that the measurements were made by different groups of observers and research teams over the years (the legend is not shown here for clarity). The red group to the right has been made members of our ALTAIR group in the past few years (we try to observe this star every month if we can). As mentioned above, the period of HS0705+6700 is about 2.3 hours, so 80000 orbital cycles represent roughly 21 years of measurements. The zero Epoch is normally defined and agreed to be a certain historical observation and sometimes a measurement prior to this is found in the literature as is the case with the blue measurement with a negative cycle number at far left. The error bars tend to become smaller as time goes on due to improvements in technology. Some of the measurements on the left will have been made from photographic plates and photomultiplier detectors, whilst CCD cameras will have been used in more recent years.

Making sense of the O – C curve

The simplest way to compute the time of any epoch is to measure the period to the best accuracy possible and then add integer multiples of it to the date and time of the first epoch. If we call the Epoch number E, we can write a formula like this:

JDE = JD0 + E x Period

This is called the linear ephemeris. JD means Julian Date which is just a way of representing a date and time as a number. As I type this, the Julian Date is 2459444.8958333335. There are many on-line Julian Date converters available to try. They are handy because you can find differences between dates by subtracting them and calculate future dates by adding days to them. This is why it is common practice to quote the period of binary stars in days. For example, the period of HS0705+6700 is 0.09564668 days and the linear ephemeris for this star system is as follows:

JDE = 2451822.75964598 + E x 0.09564668

The value of the reference Julian Date of the JD0 in the above equation is a date back in October 2000. With this, you can now calculate the date and time of any epoch in the future, or past. If everything was purely working to Keplerian celestial mechanics, every point in the above O – C plot would be on the zero line and each point would be exactly 0.09564668 days apart, so we now must address why this is not the case.

Mechanisms that can Change the Binary Period

There are three mechanisms that are known to have an effect on the period of this kind of PCEB system and these are:

  • The Applegate Effect
  • Angular momentum loss
  • The presence of a third, unseen body (or more than one unseen body).

Before I briefly explain these effects, I think you will be able to imagine that if we can calculate the sizes the first two effects, and we still find that they cannot account for the full magnitude of the O – C variations seen, then we are left with the conclusion that some unseen body must be the cause. This is the reasoning used so far by researchers in this field, and it is the way that those 16 ETV exoplanets mentioned above have been ‘discovered’.

Another thing that makes the above argument more convincing is that, when all other known effects have been subtracted from the O – C plots, what is often left over is a cyclic variation. Surely an exoplanet would leave a cyclic, periodic fingerprint behind as it goes around in its orbit?

Just a few words about the first two effects in the list above:

The Applegate Effect

A mechanism to potentially explain the eclipse time variations seen in O – C diagrams was proposed by Applegate in 1992.  This process relies on the secondary star being magnetically active, undergoing solar-like magnetic cycles which are strong enough to redistribute the angular momentum (AM) within the star. No loss of AM from the system is required. This leads to shape changes which affect the gravitational quadrupole moment. Since the orbits of the stars are gravitationally coupled to variations in their shape, this effect could explain the small changes that are observed in the orbital period. We can calculate the magnitude of the AM variations seen in the O – C plots and then compute the size of the Applegate effect. Generally, it is found that the Applegate effect is at least an order of magnitude too small to explain the O – C variations (at least this is the case with the PCEBs that we are currently studying).

Angular Momentum Loss

Note that we are talking about a loss of AM, not a cyclic variation in AM. This means that there should be a gradual decrease in the period over time as the AM is lost. There are two causes of AM loss.

The first is caused by Gravitational Radiation. This is a consequence of Einstein’s General Theory of Relativity. Accelerated masses cause ripples in the geometry of space and time and carry energy away from the binary system.

The second effect is known a Magnetic Braking which is a theory explaining the loss of stellar AM due to material getting captured by the stellar magnetic field and thrown out to a great distance from the surface of the star.

The sizes of both of these effects can be calculated. Often, like the Applegate effect, their contributions are not enough to explain the O – C variations seen.

Once this work has been done and the known effects have been subtracted, the remaining job is to calculate the number, mass and orbits of potential exoplanets that could explain the cyclic variations in the O – C. As you can imagine, this is not an easy task!

Our current work focuses on following up with lots of new eclipse timings for the 18 stars in our canon. We are interested in how the proposed exoplanets are faring against our new timings.

Further Discussions

This blog article is an overview of our work. I have not explained any details about how we observe and measure the brightness of the stars, nor how we analyse the light curves to calculate the times of minima. We have written several Python tools to do all this and maybe the next blog will go into a bit more detail.

 Posted by at 8:49 am
Sep 112015

Here are three different versions of M16, The Eagle Nebula in the constellation of Serpens Cauda. Interestingly, the constellation of Serpens is unique in that it is the only one that is split into two distinct pieces, namely Serpens Caput (the head) and Serpens Cauda (the tail). All of these images have been recently taken using the amazing telescope that I co-share with Australian amateur Jason Jennings. This scope is hosted in the ‘barn’ at the Siding Spring Observatory, Coonabarabran, NSW, Australia. I’ll write another post about the scope soon, but it is an amazing 16″ f/3.5 astrograph.

This first version is a ‘traditional’ LRGB image, meaning it has been made by taking separate images using Clear (Luminance), Red, Green and Blue filters and then combining those to make a final colour image. This should be close to how the eye would perceive the colour because the R,G and B filters pass frequencies of light similar to the sensors in our tri-colour vision system. The clear filter is used as a luminance channel and is where most of the sharpened detail resides.

As with all the images, please click on them to see a full-sized version.

M16 LRGB Version

This next version is taken using three narrowband filters. These are H-Alpha (Ha), OIII and SII. The wavelength of these filters are commonly used by astronomers because there are a lot of emission nebulae that have excited atoms in them that emit light in these wavelengths (especially Ha which is nearly always the strongest). So, to produce an ‘RGB’ image from them requires that they are mapped to the Red, Green and Blue channels of the image. I have chosen to use the ‘Hubble Palette’ which maps the SII to Red, Ha to Green and OIII to Blue. Here is the result:

M16 Narrowband Version

You will notice that the star colours are not good in the narrowband version and this is a consequence of the filter mapping and also because of the relative strengths of the three channels. So, in the third image below, I have combined the stars from the RGB image with the nebulosity from the narrowband image. Here it is:

M16 – NB with RGB stars

I’m not sure which version I prefer!

Finally, a 4th image (I lied!) taken last year with a longer focal length instrument (12″ f/9 RCOS) which shows the ‘Pillars of Creation’ in more resolution. This was also taken using the Hubble Palette which is appropriate because the iconic pillars were made famous by those fabulous images from the Hubble telescope.

The heart of M16

 Posted by at 2:13 pm

Nova Delphini 2013 – It’s official!

 General Astronomy  Comments Off on Nova Delphini 2013 – It’s official!
Aug 162013

The possible nova with the catchy designated name of PNVJ20233073+2046041 has now officially been named as Nova Delphini 2013.

It was clear here last night for an hour or so just as the sky was dark enough for me to make out the stars in Delphinus and Sagitta, so I took this shot of the new star with a 50mm lens on a Canon 60D. I’ve annotated the image so that you can where to find the Nova.

 Posted by at 7:59 am

A New Star in Delphinus

 General Astronomy  Comments Off on A New Star in Delphinus
Aug 152013

A bright Nova has appeared in the little constellation of Delphinus (The Dolphin). It is on the limit of naked-eye visibility, at roughly magnitude 6.3,  but binoculars will show it well. Here is an image I took of it this morning from a telescope in Australia, but it is well placed now for Northern hemisphere observers, and I hope it might be clear enough tonight to photograph it from here in the South of England.


Below is a map showing the constellation of Delphinus relative to Altair in Aquila and Albireo the head star of Cygnus the Swan. The position of the Nova is marked by the red circles. Click on the map to see a full size version.


This Nova, discovered by Japanese amateur Koichi Itagaki, is caused in a double-star system when material from one of the Stars builds up, or accretes on to its companion star (normally a white dwarf) until it undergoes a thermonuclear explosion and brightens very dramatically.

For those interested in finding the exact position, the co-ordinates are RA 20h 23m 30.7s, Dec +20° 46′ 03″


 Posted by at 3:59 pm

Zooming in on The Ring Nebula

 Deep Sky, General Astronomy, Zoom in on ... series  Comments Off on Zooming in on The Ring Nebula
Aug 092013

What was it about The Ring Nebula (M57) that inspired me to put together this little blog article? I guess it must be that it was the first telescopic ‘deep-sky object’ that I ever learned how to find and observe . (I didn’t call them deep-sky objects back then and the brighter wonders such as the Andromeda Galaxy and the Orion Nebula which are visible to the naked eye don’t count! ) I remember using my 60mm Tasco refractor back in 1971, as an 11 year old to look at this lovely object, and was amazed by it. Within a few months I would see it in Patrick Moore’s 12″ Newtonian Reflector – well what can one say – incredible!

It was always likely to be M57 (out of many other objects) because it is really easy to find, located as it is between two stars in the ‘parallelogram’ of Lyra the lyre. Also, Lyra itself is easy to find because of its brightest star Vega, and the fact that Lyra is a compact constellation right next to it.

Before we go zooming in on M57, what exactly is it? It is a planetary nebula, so called because, at first glance,  it presents a planet-like disk to an observer through the telescope. However, instead of being at Solar System distances, it is roughly 2,500 light-years away and was formed when a dying red-giant star blew out its outer layers before becoming a white dwarf. Such a fate will probably befall our own Sun in about 5 billion years! Now, we see the remaining shell of ionised gas as it expands into the interstellar medium.

So, the idea behind this blog is to locate M57, and zoom into it using images that go from wide-angle camera shots to high resolution images from large telescopes. First, I’ll transport you back to my world of the early 1970’s by showing this scan from the wonderful star maps at the back of the classic Norton’s Star Atlas. I still reach for this book when I need to remind myself of various bits of the night sky! Here is the scan, I added the insert showing Lyra at a larger scale, but the map itself in very evocative to me, covered with the rubbed-out tracks of pencil-drawn trails from long-gone Perseid or April Lyrid meteors. You will see M57 indicated between the stars β (Beta) and γ (Gamma) Lyrae at the bottom of the parallelogram shape (which I have outlined).


Now, on to the first image. I took this back in early June this year using a Canon DSLR camera and an 18mm lens, giving a nice wide field view. (as with all these images – click on them to see them at full size, then click again to return). I have indicated a large green triangle which is known as The Summer Triangle consisting of the 3 bright stars Vega, Deneb and Altair. I have also annotated the cross shape of the constellation of Cygnus the Swan which is getting rather swamped by the Milky Way in this picture. You can see Lyra near the top.


The next image is only a bit more of a close up Lyra, taken with a 28mm lens this time. I have added the names of the two stars that straddle either side of the ring nebula. As you can see Beta Lyrae has the proper name of Sheliak, and Gamma is called Sulafat. Also annotated are a few of the other brighter stars in this field – Albireo is the head (or beak) star in the cross of the Swan. Many of the proper names of stars in use today come from an Arabic origin and Sulafat comes from the Arabic for ‘turtle’ or ‘tortoise’ as it seems that most fine harps (or lyres) were decorated in tortoiseshell.


So, let’s zoom in a bit further. Next I changed to a 50mm lens and have also added an insert to this image. The insert is at the full resolution of the image whereas the rest of the picture has been much reduced in size to get it on this page. Now we can actually see the Ring Nebula! It’s pretty small as you can see, in fact it is approximately 3.5′ (arc-minutes) across which is, roughly speaking, only one tenth the apparent diameter of the Moon. Note the use of the word ‘apparent’ there; The true size of M57 is some 3 light-years across!


Now the last camera shot, before moving to a ‘proper’ telescope (telephoto lenses are telescopes really, but you get the idea). This time, I used a 200mm lens and I have cropped out the Sheliak/Sulafet region. Now we can see the ring and can understand why this is known as a planetary nebula.  It certainly confused its discoverer. French astronomer Antoine Darquier de Pellepoix in January 1779, reported that it was “…as large as Jupiter and resembles a planet which is fading.”  This is a good description as Jupiter is typically about 45′ across, but much brighter of course! Another Frenchman, Charles Messier, independently found the same nebula later on in the same month while searching for comets. He entered into his famous catalogue as the 57th object (Note that the main reason for Messier’s famous list was so that he would remember these ‘fuzzy’ objects and not confuse them with Comets which were his main interest).


The following image was taken with my Celestron C11 telescope and an ATIK 383L cooled CCD camera. I used a filter called an H-Alpha filter which passes light in a very narrowband of frequencies. I was after the outer shell of M57 – something I had never really seen in older photographs, but nowadays it is commonly captured by amateur astronomers using sensitive CCDs. It does require long exposures to bring the faint outer shell out, and I stacked together several 20 minute exposures to reveal it here.



The previous, rather noisy, image is certainly not my finest moment! I really don’t have a good telescopic image of M57. So to finish this article with a splendid image, I asked Robert Gendler if I might use one of his (for those of you who don’t know, Robert is one of, if not the best deep-sky imagers and image processors in the world) . He kindly suggested that I use this incredible image of M57. For more information about this particular image, and about M57 in general see Robert’s page here:


That’s it for my take on M57. Maybe I’ll make the Zoom into theme a regular feature here, so watch this space!


 Posted by at 4:23 pm

Meet The Spodies!

 General Astronomy  Comments Off on Meet The Spodies!
Jun 072013

Astronomers like to blame ‘Spode’ when skies are cloudy, or when things go wrong. I’d like to introduce characters called ‘The Spodies’ who are the brainchild of Roger Prout. Roger was one of the founding members of the South Down Astronomical Society (SDAS), based in the Chichester area of West Sussex. (The South of England).

Many years ago, as a teenager in the 1970’s I was assistant editor to John Mason, producing a magazine called ‘Supernova’. This was the magazine of the SDAS, and it was widely regarded as the best astro society magazine in the UK at that time (well we certainly thought so). Here’s what a typical cover of Supernova looked like (we even had photos on later editions which was very rare at the time!)


Here’s one of the cartoons. This one showing the Spodies craftily directing clouds. This shows the Selsey peninsula and the telescope in question will certainly have been one belonging to Patrick Moore! (Patrick being a good friend of the SDAS).


Two more are shown below (click on them to see full-size). One refers to the state of British astronomy and the discovery of  a nova in Cygnus by Japanese observer Minoru Honda (V1500 Cyg – Nova Cygni 1975). The other was topical at the time when Jupiter’s Great Red Spot all but disappeared.


Finally, meet the lovely Andromeda. Another of Roger’s creations, she graced the pages of Supernova from time to time. I always remember this particular cartoon, and I still chortle when I see it.

 Posted by at 12:07 pm

Jupiter Near Opposition

 General Astronomy, Jupiter, Planets  Comments Off on Jupiter Near Opposition
Dec 052012

Jupiter reached Opposition at about 1am UT on the morning of 3rd December 2012. Here are two images of different aspects of the planet. The one with the GRS was taken on the night of the 3rd and the second image taken on the 4th, with slightly better seeing conditions.

Opposition means that the Sun, the Earth and Jupiter are in a line. The Sun is therefore shining directly on to the face of Jupiter as we look at it. It also means it is closest to us and hence the disc is the largest it gets in this apparition.


 Posted by at 1:00 pm

BBC Sky at Night 55 year party

 General Astronomy  Comments Off on BBC Sky at Night 55 year party
Apr 172012

What a great event! I was very honoured to be invited to the party at the BBC Broadcasting House to celebrate 55 years of the Sky at Night. The remarkable Sir Patrick Moore has been presenting this excellent programme since it first aired on 24th April 1957 – an achievement unparalleled in broadcasting history, and the longest running TV programme ever.

Patrick is the greatest communicator of astronomy in history – fact! Forget the web, twitter, facebook and the rest, (says he on his blog!), this man has inspired more people to become astronomers, or to simply love the subject more than any other influence.

The party itself was brilliant. It was great chatting to the likes of Sir Tim Rice, Brian May, Jon Culshaw, Sir Terry Pratchett along with plenty of dedicated astronomers, both professional and amateur. I have shamelesly included some celeb pictures below! Thanks also to Pete Lawrence, Damian Peach and Ninian Boyle for your excellent company and banter during the trip up from Selsey.

 Posted by at 8:58 am
Apr 052012

A glorious sight in my 75mm APO refractor telescope last night. The night of April 3rd was cloudy when Venus was nicely in the main cluster, but I was still able to get both Venus and the Pleiades in the same field of view last night.

Venus is massively over-exposed here, but I like the effect of the burn-out and spikes it causes. You can just see the faint nebulosity around the main stars in the Pleiades. This image is the result of stacking 20 exposures each of 20 seconds using my ATIK 383L CCD camera and a white luminance filter – through the Pentax 75 APO.

 Posted by at 7:18 am