In November 2025, I was awarded a 5-year SNSF Starting Grant to lead a large project on understanding Neptune-like planets. Here is a brief summary of that project and what will be involved.

Thanks to surveys like Kepler, we now know that most stars like the Sun host a place between the size of Earth and Neptune, and most of those planets orbit close to their stars – usually with orbital distances closer even than Mercury. Without such a planet in our own solar system it is hard to understand what such planets are made of and how they might form.

But we now have a remarkably detailed & growing cache of observations for such planets. This has been helped by multiple developments in the last decade. On the ground, our high-resolution spectrographs have become even more powerful (e.g. ESPRESSO, Maroon-X, etc), enabling precise masses for planets down to a few times the mass of Earth. In space, NASA’s TESS spacecraft has rapidly found thousands of planet candidates, especially Neptune-like planets orbiting nearby & bright stars, and ESA’s CHEOPS satellite has been instrumental in following-up many of these (see e.g. my last two research posts on here). And last but not least, NASA’s JWST has for the first time revealed the contents of the atmospheres of a large number of small Neptune-like planets.

We now have dozens of Neptunes & sub-Neptunes with precise masses, radii and atmospheric measurements. This has resulted in the first estimates of bulk compositions for some of these low-density planets, revealing some exo-Neptunes which are rich in water, others that seem depleted in water, and yet more where their atmospheres appear too thick for precise measurements.

Despite these measurements, there exists much uncertainty in exactly where these planets formed and what they are made of. Some theories suggest they form in the outer ice-rich regions of their solar systems and then migrate into their current locations, while others suggest they grow close-in from principally rocky material. And the current set of observations are extremely patchy with biases toward the observation of certain planets (such as close-in planets around M-dwarfs).

As I see it, there are multiple issues with the current observations of exo-Neptunes:

1. Planets at longer orbital periods are missing because they have not been found via consecutive transits in TESS, and their confirmation takes time
2. A fraction of detected planets have not yet been sufficiently characterised (in e.g. mass using RVs or atmospheres using JWST), leading to incomplete and biased target lists
3. Radii, masses & atmospheric information is derived on a per-target basis with different extraction and retrieval routines, leading to results which cannot be directly compared
4. There has been no study of planetary demographics to measure the underlying occurrence rate of compositions derived via atmospheres

The project is therefore designed to fill these knowledge gaps. Starting from a golden sample of around 10,000 stars for which sub-Neptunes can be detected & characterised in both radial velocities (RVs) and JWST, the plan is to uniformly detect and characterise all Neptune-like planets orbiting these stars. The first key step is to use advanced techniques (e.g. machine learning) to detect missing exoplanets around these stars in TESS photometry as well as to assess how likely planets of given period & radius will have been detected. These planets will then be confirmed via CHEOPS observations if necessary. The next step is to both measure these planets masses as well as to search for additional planets in the system using RV observations (and, in some cases, transit timing variations – TTVs). This will produce densities for the exo-Neptunes as well as reveal the system architecture – e.g. whether the planet is alone or in a multiple system. The next step is to take transmission spectra of the ~100 planets within this sample to uniformly derive atmospheric abundances for the sample. This will be a mix of existing (e.g. archival) observations as well as new observations performed to make sure all planets have similar datasets.

The final goal is to take all of these observations and analyse them together in order to produce maps of exoplanetary compositions as a function of planetary age, architecture, etc. This can then be compared with theoretical predictions in order to help answer key open questions in planetary formation and evolution.

To apply for a PhD in the group focused on the detection of new exo-Neptunes (see below), head to the University of Bern job portal (deadline: 30th Nov 2025). In late 2026 (deadlines early summer 2026) I will also hire an additional PostDoc working principally on measuring precise masses and system architectures (using RVs & TTVs), as well as a PhD focused on atmospheric observations of exo-Neptunes.

Phase I – detection

Detecting sub-Neptune exoplanets forms the first part of the project and the thesis of topic of the first PhD student (to start in early 2026, ideally before April). The goal is to reliably recover all of the transiting Neptune-like planets in TESS data for golden sample of ~10,000 nearby & bright stars. These will form the starting point for the characterisation of these planets in masses & atmospheres. The vast majority of transiting planets have already been detected by one techniques or another, and many already have RVs or even spectroscopy. But some are missing – typically those on longer orbits with only a handful of transits, or orbiting bright or active stars where traditional detrending techniques fail. In order to reliably detect these we need a different approach which better able to find transit-like features in noisy TESS lightcurves. As these candidates may not have clear orbital periods if the transits are non-consecutive, one next step will be to help pin down the true periods via CHEOPS observations. Additionally, we want to be able to assess how common a particular planet type is, which requires an answer to the question “around which stars could this type of planet have been detected?”. So a secondary part of Phase I will be to reassess the occurrence rates of Neptune and sub-Neptune planets. This is key to finally being able to assess the occurrences of exo-Neptunes as a function of planetary compositions and formation pathways.

We have a big press release today on the discovery of the HD110067 system, involving more than a dozen institutions across the US and Europe. That focuses on the science and the broader picture of these new planets. But I thought I would share a more personal viewpoint of the idiosyncratic (and somewhat unique way) of exactly how we found these planets.

The story begins in lockdown 2020. The Transiting Exoplanet Survey Satellite (TESS) had launched 18 months previously and I was at MIT working with the TESS science team. This space telescope scans a large fraction of sky for a month looking for the hints of transiting exoplanets – planets whose orbits pass between us and their stars, causing a brief dimming in their starlight which we can detect. The depth of the dip tells us the size of the planet and, when we have consecutive transits, we also get the orbital period of the planet.

One of my roles at MIT was to help run the candidate vetting process. Two separate automated pipelines churn through the data from all the stars TESS has observed and spit out planet-like signals. Our job is to look through the reports and determine which ones really were planetary (to become “TESS Obejcts of Interest”) and which ones weren’t. In late April 2020, the first hints of planets orbiting HD110067 arrived in the form of the PDC vetting report, and it immediately piqued my interest given the star was extremely bright. The data also showed a handful of clear U-shaped transit signals – good signs of planets – but the automatic pipeline itself didn’t do a great job linking up the various transits. One unfortunate problem with TESS’ observations is that it typically only looks at a patch of sky for 27 days before moving on, so even planets with 15 day orbits might not show two consecutive transits in each given sector. However, ESA’s CHEOPS satellite (the other half of my two-mission fellowship) is a targeted telescope, so we tell it where to point, and in this case I immediately wondered CHEOPS could help.

But the first thing to do was to model the data and see if I could figure out exactly what was going on… There seemed to be five clear transits, but from two or more planets. and so we needed to play a game of “match the dips” – which transits are from the same planet? Two relatively shallow dips 9 days apart seemed to link together well, as did two other deeper dips 5.4 days apart (although they should have produced a third transit, I thought this might have erased by noise). The fifth and deepest dip didn’t seem to link with anything else, so I thought this might just be a single (or mono-) transit. With this solution in hand, we got TESS PI George Ricker’s approval to speed up the TOI announcement process, and immediately gave the transit periods to CHEOPS for observations which… rejected them. It turns out we were about 10 days too late and HD110067 had just slipped out of the observability region of CHEOPS… Dammit!

Two years later

In the mean time, I worked on other projects involving TESS & CHEOPS, finding a bunch of other interesting planets. But by the time spring 2022 came around, I realised that TESS was about to observe HD110067 again! I was interested to see if our friend the monotransit would show her shadow again, as these planets with two transits separated by a large gap (which I have coined “duotransits”) were exactly the types of planets my CHEOPS programme was designed to follow. Luckily the guys at MIT had managed to create a rapid release pipeline for TESS data called TICA, and thanks to some other CHEOPS projects, we had even managed to convince them to release each TESS orbit extremely quickly. This was just the thing we needed, as HD110067 would set from CHEOPS’ view only a few weeks after TESS finished observing.

So I grabbed the TESS data from the new sector, and I have to say I was shocked! There were so many transits! I thought there could be three planets in this system, but it looks like I had been very wrong… After looking a bit more, I found our friend the planet at 9.11 days – so we had at least one planet with a confirmed orbit. But the 5.4 day planet was conclusively ruled out. And there were another four transits in these 14 days of TESS data which appeared new!

Time to play a “Pair Matching” game…

Single transits are extremely tricky to deal with – the potential periods are effectively unlimited. But if you can find a second transit, then you drastically reduce the possible periods from infinite down to a few dozen of “aliases”. This is the game I had been playing with other TESS candidates, and it was the game I wanted to play with HD110067. But it’s a lot easier when you have only 2 transits as opposed to 12… How can we figure out which transits are from the same planets? Well, the best way was to play another “pair matching” game, just like for kids. Take the shape of a transit in 2020 and match it with the shape of a transit in 2022. Of course, I did this by fitting models to the transits and comparing the resulting parameters rather than eye-balling it, but the end result is the same – three of the 2020 transits seemed to match with three of the 2022 transits, while two more didn’t seem to match at all.

So, now we had three “duotransits”, we could compute those possible periods, and start using ESA’s CHaracterising ExOplanets Satellite (CHEOPS) to hunt for the true periods. Thanks to my position within the CHEOPS team, I could start checking possible periods immediately after TESS stopped observing. We also got the second orbit of TESS data, which didn’t bring any more unexpected transits (we had enough), but solidified our knowledge of the 9d planet. A little bit later we were also able to improve our lightcurve using algorithms developed by Andrew Vanderburg, and this revealed that one of the planets we thought was a duotransit (i.e. with only two transits) actually had another transit hiding in noise, turning it into a planet with a 13.5d orbit – two planets solved, four more to go!

After many observations with CHEOPS for these possible aliases, we finally got lucky in late April and caught a third transit of planet d, with an orbit of 20.5 days! Half of the planets now had orbits – we were sure three more planets were hiding in the system with periods beyond 20 days, but for the moment we weren’t sure where they might be. However, it was at the point that I got the CHEOPS data for this new planet and solved it’s period that a cog clicked into place – the periods for planets b, c and d all appeared to increase by 50% each time – a clear hint for “resonance”.

Resonance is like a gravitational dance – the subtle influence of interior and exterior planets (or, in the case of Io, Europa & Ganymede, moons) lock their orbits into tight ratios and maintain this intricate balance, sometimes over billions of years. The planets may slightly oscillate around these precise integer period ratios (2:1, 3:2, & 4:3 being the strongest), but each time their periods drift the gravitational pull of a neighbour brings them back into lock-step. We know of a few key systems like this – TRAPPIST-1 & TOI-178 for example – but even they have certain planets which break from a perfectly “first order” resonance.

I also very quickly realised that the last remaining “duotransit”, which had more than 30 possible periods, was surely at the next position in the chain. And, sure enough, one of those resonances matched almost exactly (with a precision of <0.1%) a 3:2 orbital resonance with planet d. My colleague at the Observatory of Geneva, an expert in resonances, confirmed that the inner three planets were a stable resonance, and that planet e with a 31 day period was also stable and by far the most likely.

So, with the detection planet d, the dominoes started to fall and the next planet in the chain was solved. But the next two candidates didn’t have the luxury of producing two transits in the TESS data… So how to proceed. Luckily, Adrien came up with a plan to test every possible combination of resonant orbits for the outer two planets in order to test two things. The first was whether a planet at that orbit would have been spotted by either TESS or our subsequent CHEOPS observations, and the second was how far that planet was from a resonant equilibrium – effectively planets far from equilibrium are unstable over long timescales, and therefore far less likely to persist.

After trying 2:1, 3:2, 4:3, 5:4 & 6:5 orbits for each of the two outer planets (a total of 25 cases), we found that, remarkably, if the planets were resonant then there was a single likely solution – an orbit of 43 days for planet f and an orbit of 54 days for planet g. All other possible orbits were either ruled out by our data, or appeared far from the stable resonant equilibrium. At this point, we thought it was extremely likely that we had solved the whole system. But convincing the community (and a referee) was not going to be so easy… So we needed independent evidence for the planetary orbits.

Ground-based telescopes pick up the baton

By this point, unfortunately, HD110067 was no longer visible to CHEOPS. But there was a chance that we might be able to spot the outer planets in data from ground-based telescopes via two methods. The first was to look at radial velocities, which our Spanish colleagues had been gathering using the HARPS-N and Carmenes telescopes. These spectroscopic measurements can precisely measure the tiny push-and-pull caused by a planet orbiting a star, so maybe we could find planets f & g in that data. However, HD110067 is a pretty active star, and the signal from stellar activity dwarfs that of the small planets in this case. However, thanks to some state-of-the-art activity modelling from Oscar Barrangan and Andrea Bonfanti, we were able to recover the signals from three of the six planets, including planet f at 43 days. This was a clear, independent sign that there was a planet at the orbit we predicted.

The second independent sign came via a global campaign of ground-based telescopes at the time of a predicted transit of planet f, which I coordinated. Thanks to colleagues with access to the MuSCAT-2 & -3 telescopes, the Saint-Ex telescope, the Tierras observatory, the NGTS telescope array of 10 telescopes, and a DDT on the LCO network. All these multiple observatories spread across the globe sent their data back to me and, although no single observatory found a clear-cut transit signal, careful analysis of the entire dataset showed a clear hint of a transit at the expected time! So it looked like the resonant orbit of planet f was a certainty.

But planet g still only had a single TESS transit, and no hint in RVs… So we were resigned to publish the paper like that: effectively with five and a half planets. Until, at a CHEOPS meeting, Rafa Luque, who we had tasked to lead the paper, showed us a plot that Josh Twicken had made, which had re-analysed some “missing” TESS data from 2020, right when our orbits predicted transits of both planets f & g… And there they were – two transits perfectly matching those which we had seen in 2022, but which had been masked due to poor-quality. This was a wonderful confirmation of Adrien’s predictions, with the two planets being once again less than 0.1% away from perfect resonance orbits.

With that additional TESS data, we had a confirmed system of six resonant transiting planets. The system is by far the brightest resonant system of transiting planets on the sky. The architecture of the system is also unique – it is only system with six planets all in first-order resonances (the strongest), and their orbits are the closest to perfect resonance of any such system. HD110067 is also now the brightest star to host a transiting multi-planet system with four or more planets, and it could even be the brightest we will ever find. The six planets are all sub-Neptunes with radii between 1.9 and 2.9 times that of the Earth. We also know they are low-density, likely with large amounts of hydrogen-helium gas (or possibly steam). This makes them ideal candidates for JWST spectroscopy, although the star is if anything too bright for JWST making many instruments saturate!

But I am most looking forward to the next few years – CHEOPS will continue to monitor the system looking for transit timing variations – the hints of gravitational tugs between planets which can help tell us the masses. I also lead a successful monitoring proposal on the star with CARMENES and HARPS-N which should independently improve our precision on the planetary masses. There is also a very real possibility that the resonance chain is not finished at six planets. And, with our current data, we do not really know what might be lurking beyond planet g on orbits of 75 or even 150 days, although we know they should transit as the system is extremely edge-on. And if that is the case then each additional planet will be cooler and closer to earth-like temperatures… Only time will tell for this brightly shining new star of exoplanetary science.

From the dawn of modern astronomy (1710):

From the dawn of the Exoplanet age (1995):

Today we announce four new planetary systems which were discovered via a combination of TESS and CHEOPS photometry through a project I started on the CHEOPS GTO (see our ESA and Swiss press releases).

The problem with TESS

TESS is surveying the whole sky to find new transiting planets around bright stars, and it has done a fantastic job in the past few years – the vast majority of characterisable planets around stars brighter than V=9 come from TESS. However, because its observing strategy involves observing much of the sky for only 27 days every two years, planets on periods longer than around 20 days are unlikely to produce consecutive transits. Instead, what we have seen with TESS, is that many planets produce single transits during TESS observations with large (often 700+ day) gaps between transits. This is hugely problematic as it means the orbital period is still poorly constrained and follow-up characterisation efforts – for example constraining planet mass through radial velocities or atmospheric composition through transmission spectroscopy – are impossible.

This is where ESA’s CHEOPS satellite can step in. When two transits are observed this constrains the orbital period to a range of periods, usually 30-40. By modelling those transits and extracting as much information as possible from the star, transit shape, and planetary system (using my MonoTools code), this can be further reduced to a handful of possible so-called period aliases – we know that one of them must be the true period, but not which.

CHEOPS to the rescue

So how can we recover these lost planets? By using ESA’s CHEOPS satellite. This 30cm ESA telescope is far more precise than TESS, but targets specific stars. But we can target these interesting stars with long-period planets during the times when we expect the transit of one of the possible periods. We chose a selection of candidate planets best-suited to CHEOPS – planets producing shallow transits impossible to spot in ground-based telescopes (i.e. planets smaller than Neptune) orbiting bright stars.

This began a game of hide and seek between the planets and CHEOPS – we monitor the stars and check the data hoping to see a transit which would confirm the planet and its period… but more often than not we would see a flat line, ruling out that orbital period, allowing CHEOPS to move onto another one.

The four systems:

HD 15906 c

This star, much cooler than the Sun, was seen to have a single transiting planet on a 10d orbit in TESS. However, looking at the data, I spotted the sign of a second transiting planet producing two transits. We hunted the true period, and revealed an outer planet on a 21d orbit. This paper was led by Amy Tuson and published in MNRAS.

HD 22946 d

This bright sunlike star was already confirmed to host two transiting short-period planets in a previous paper. They also identified a single transit of an outer planet, but once again we spotted a second transit in the data, and went after it with CHEOPS to find the true period. We were able to confirm it as 47d for the outer planet d, as well as improving the radii of the inner planets (which due to a bug had incorrect values in the previous paper). This paper was led by Zoltan Garai and published in A & A.

TOI-5678 b

In this case, we had been successful with previous systems and therefore decided to try the same technique with slightly larger planets but using less CHEOPS data – hoping to rely on only a fraction of the transit to confirm the planetary period. So we scheduled short observations at the aliases for the Neptune-sized planet TOI-5678, and managed to catch the ingress – the first half of the transit – confirming a 48 day period for this planet. Solene Ulmer Moll, who led the paper, was also able to observe the planet with high-resolution HARPS spectra, with the radial velocities providing a mass. The planet was published in A & A.

HIP 9618 c

For this bright star, TESS initially spotted two transits which were thought to be from an 11d planet, but those transits were actually from two different planets. The inner planet transited again three times in two later TESS sectors, revealing it to have a 21d period, but the outer (and slightly smaller) planet transited only once more. As for the other three systems, we quickly turned to CHEOPS to seek the true period, finding a beautiful transit on the fourth observation. This confirmed a 52.5d period. HARPS-N, SOPHIE and CAFE radial velocities also constrained the masses for these planets, showing them to be low-density mini-Neptunes about ten times the mass of Earth and 3-3.5 times the radius. The RVs also revealed that the system has an outer brown-dwarf companion in the outer reaches of the system. I lead this paper in MNRAS.

It’s not always obvious exactly how to proceed, so here’s a quick guide.

1) Fix compilation problems on overleaf

Even though overleaf might still be able to compile a pdf with a few orange/red warnings, arxiv definitely will not. So click on “logs and output files” and go through the bugs & warnings one-by-one fixing them.

2) Download the source

You’ll need a .zip file for arxiv

3) Recompile the source on your computer

Unfortunately that .zip file isn’t usually enough (especially if, like most of us, you use bibtex). So you’ll have to unzip the src.zip and recompile the source. Of course, in order to compile the paper, you will need to have latex locally running on your computer, so if you dont (i.e. if running “pdflatex” in the command line give you nothing) then download latex from here https://www.latex-project.org/get/

If you now have latex, get the command line up, cd into the (unzipped) source folder, and now run pdflatex [your .tex file], followed by the bibtex [your .bib file], and once again pdflatex [your .tex file]. Verify that the pdf that is generated looks good and has the correct references, and that a .bbl file has been created. If so, you need to re-zip the folder.

4) Upload to arXiv

Now you can start a submission to arxiv. You’ll need to choose the license – I haven’t read much into this, but I usually choose arxiv’s own license for this. Then upload the source and hope that nothing breaks. For the abstract you can copy it from overleaf (most peoples computers should compile any latex maths in the abstract) but make sure to remove any macros and comments.

For the comments, it’s typical to put the paper status (is it submitted, accepted, etc?), the number of tables and figures, and any other accompanying links/data/etc.

Some extra points:

• Do not upload the proof-corrected version. The journal you submitted to has rights to that – you only have rights to the pre-corrected (i.e. preprint) version.
• Go through the paper comments and remove anything… silly. Otherwise the overheard on astroph bot might find it.

Occasionally I have wondered which exoplanets are most loved by astronomers. One way to find the answer is to pit planets against each another and see which one survives… something my podcast exocast does every year in the #ExoCup!

But another way to answer this question is to look at the trends in the published literature – which planets get cited, mentioned and observed most often? But doing that would mean scraping through every single published astronomical paper for the last fourty years for every single one of the 4000+ exoplanets…

So, with the help of ADS, that’s exactly what I did. And here is the result, plotted using bokeh as a function of time vesus combined exoplanet mentions. (Click here, or scroll below, for the full interactive version)

And the most-cited exoplanet? It’s a classic – the first world ever to be seen transiting, and the first to have its atmosphere detected – HD209458b. And, as always just following it, is HD189733b – the second brightest transiting hot Jupiter, and the first planet to have its surface mapped.

There are other interesting battles for the Top-10. For example, which is the best planet around an M-dwarf? GJ1214b and GJ436b (both transiting Neptunes) are locked in a tight battle for third place!

And what is the best directly-imaged planet? After a long timebehind Kappa And b and HR8799, Beta Pic has spent the last 5 years as the king of Imagers. But 51 Eri b and PDS 70b are steeply rising to challenge for its crown…

There’s a cluster of planets apparently referenced pre-1995, but the majority of these are simply errors in my paper-scraping code. For example a paper with the sentence “stirrups and coat” would flag the planet “Ups And c” (41 mentions since 1993).

The legend is interactive, and if you want to best compare the paths of two planets, you can use the legend labels to “mute” all the planets and then select only those you are interested in. Click here for a full-screen version.

Move aside Tabby’s star – a new enigmatic star observed by K2 looks to have taken the title of the “most mysterious star in the galaxy” – HD 139139. Here I’ll try to explain why it’s just so weird.

The Basics

First, a few basics – we use satellites like NASA’s Kepler mission to find exoplanets, planets orbiting other stars, by monitoring the brightness of stars. In some systems, the planets pass in front of their stars, blocking some starlight from reaching our telescopes – a “transit”. The amount of missing light (the transit depth) tells us the size of the object crossing the star, and the length of that dip (the transit duration) is usually related to the speed of the body – i.e. its orbital velocity.

The key thing about these transits it that they are periodic – every single orbit of the exoplanet, it produces a transit. So when we go hunting for these systems, we find repeated dips, and the “light curve” (the change in light over time) looks like this:

Here you can see the biggest planet transiting, like clockwork, every 10 days. There are also two more planets – one causing four transits, and one producing just two. But this light curve shows unmistakable periodicity. Of course, we don’t check that by eye – we run a periodic search on each light curve which tests each period and checks for regular dips.

Of course, when a telescope is staring at hundreds of thousands of stars, it also spots other things passing in front of stars – starspots on a distant star’s surface for example. Or so-called “exo-comets” where we see the comet’s tail cross the star producing an assymettric dip. In very rare cases, planets themselves can end up mimicking comets as we watch them get evaporated by their parent star, jetting dust around the small planet, and causing occasional periodic transits which come and go as dust get created and then burned off.

Other interesting “dips” occur for young stars, which are still in the process of forming. Here there is gas and dust aplenty being thrown around by various processes (planet formation, stellar wind, accretion of material onto the star along magnetic field lines). These produce bizarre assymettric and irregular dips in the light curves of young stars (we call these “dipper” stars), and are something that I have experience in studying.

HD 139139

So you’ve seen what a typical exoplanet lightcurve looks like above. It’s time to show you HD 139139’s lightcurve. It was observed for 80 days by NASA’s K2 mission, with exquisite precision – the typical errors on each point is 30 parts per million (0.003%), and was published by Saul Rappaport et al in June 2019 after being detected initially by citizen astronomers.

It looks extremely similar to those I showed above – a forest of dips. 28 in total, most with similar depths of around 200ppm. And the dips themselves look similar too – they have the same distinctive U-shape as planets:

But, the more you look and play with the lightcurve the more you see something troubling: they are not periodic. AT ALL. Here’s what happens when you run a “period match” test against a typical planet (K2-110, left), compared what happens when you run it on this lightcurve (right)…

There’s really nothing. There is no way to link these dips, even slightly. Rappaport et al even tried to relax the necessity of perfect periodicity (sometimes so-called TTVs between neighbouring planets pull them away from strict periodicity), but even this seems to not work… In fact, when you compare the timings of the dips, instead of matching regular periodicity of a planet, they match very well what might happen if random times were simply being pulled out of a hat – it really is a “Random Transiter”.

Also interesting is that most of the dips appear similar sizes (200ppm means objects about 50% larger than Earth), except for a single observation of roughly double this. This suggests that the objects have mostly the same size. The duration, however, seems to vary wildly.

The team also did extensive work to make sure these were real signals, and not some kind of bizarre instrumental problem. But the dips really do appear to come from this star.

I say star, there are in fact two stars superimposed together in the data – a solar-like G-type star about 1.5Gyr old, and a slightly smaller star about 3 arcseconds away which seems likely to be connected to the brighter star in a binary system.

So what off Earth is going on? In the paper, they go through a series of ideas for what could be happening, but the clearest answer is: nobody knows…

Possible explanations

A plethora of planets? – if enough planets orbited the star, they could all be responsible for a single (or possibly two) dips. However, not only is it extremely improbable that 14+ planets would all be in-transit and all have similar radii, they would be certainly on unstable orbits to produce such similar transit durations. This is also true if we suggest some planets are orbiting the smaller star B. And even if we assume the planets “wander” around a true period due to TTVs, they simply cannot wander far enough or be numerous enough to explain the data. So we can rule out classical planets.

A disintegrating planet? – As I mentioned above, a handful of objects have been found with dips that come and go, caused by a small planet in the process of being evaporated sending out a cloud of dust which is eventually blown off. However, in these cases, there is still some degree of periodicity – the body may not cause a transit every orbit, but every time it does it should happen at the same point in the orbit. Also, HD139139’s dips are seen to happen at a minimum 5 hours apart… but such an orbit is likely unstable, and also incompatible with dips that last longer than 5 hours.

Dust-emitting asteroids? – In some systems, bizarre aperiodic dips have been spotted that appear to be due to planetesimals that are undergoing evaporation (much like our disintegrating planet idea, but with multiple bodies). This is true for the young star RZ Psc and the old white dwarf WD-1145. To me, this is the scenario that looks most similar – the dips can be transit-like but are not periodic (although in the case of WD-1145, they are often followed for more than one orbit, giving a periodicity). However, there are some problems with this idea – namely the fact that all of the transits here are the same depth! These clumps of asteroid should not all be producing exactly the same dust clouds in terms of both size and density – they should be far more variable. Similarly, how are these asteroids (all on different, wide orbits) at just the right orbit to produce so much light-blocking dust?

Planets in a Binary system? – One way to avoid periodicity of planets is to place them in a binary star system. I guess in this case we mean in a triple system (with one of the two stars we see being a tight binary & a planet and the other not involved). In this case, the stars are moving, therefore not every orbit produces a transit, and they can have different durations (but often the same depth). Sounds familiar right? There are two ways to do this – place one planet around both stars, or place a planet around a single star. However, both options would require extremely short periods for both the planet and the binary to make so many dips, and the team could not find a stable system for either case which matched the data, and the radial velocity measurements rule out this being a binary system.

A young “dipper” star? – These are young stars with random clumps of dust raised from the circumstellar dust disc into our line-of-sight, blocking up to 50% of the starlight. However, even if this dipper behaviour is happening on the fainter companion, it do not resemble at all what is observed from the Random Transiter – there is no periodicity, no out-of-dip variability, and all evidence suggests the star system is old and doesn’t have any obvious dust disc (which would show up as excess infrared light). Also, the assymetric dips of these “dipper” stars really shouts “clumpy dust” to me, as does the lightcurve of the bizarre Tabby’s star. The dips of HD139139, on the other hand, appears far more ordered and “planet-like”. However, these dipper stars are poorly understood (as I know well) and it is entirely possible we have found a bizarre new member of the group.

Short-lived star spots? – Another poorly-understood part of exoplanetary science is that we don’t know exactly what stars are capable of. On the Sun starspots last weeks, and sometimes multiple stellar rotations. In this system, the star appears to spin every 15 days, but maybe there’s some rare process where a starspot could bubble to the surface, depress starlight for a few hours, and then dissipate entirely.

SETI? – This option is not mentioned in the paper, but it is a system that is sure to interest those who are certain Tabby’s star is actually an alien megastructure (it’s not). If anything, the coherent dips in this system look more like solid structures than the incoherent wandering of flux during the eclipses of Tabby’s star (though the orbital periods the transit durations suggest don’t really make sense for any solid structure). Some also suggested searching for pi or prime numbers in the signals, but if an alien was trying to get our attention and had the ability to build structures 1.5 times larger than Earth at random orbital periods why wouldn’t they just, you know, send us a radio or laser pulse? To me, it makes far more sense that this system is some previously-unknown natural phenomenon instead of relying on an “alien of the gaps” logic, but there will always be those who want to believe.

Summary

The only thing we know for sure is that HD139139 (or its neighbor) has something weird around it. Exactly what that is, we can’t yet say.

The remarkable thing is that, unlike the Tabby’s star case (which imho always seemed like it would be clumps of dust), the symmetric and similar-depth transits here could be equally stellar activity, dust clumps, or bonafide terrestrial planets…

For me, I would lean towards one of the “evaporating asteroids” or the “starspots” hypotheses. But exactly how those scenarios would produce such similar depth eclipses is a real mystery… One thing that will help is, as always, more data – we really need photometry in different colours, which will tell us something about the composition of the objects crossing the star.

But until a definitive theory arrives, we are going to live with that thing every scientist both loves and dreads – the uncertainty of not knowing.

My most-cited work is probably not a peer-reviewed paper (to my shame), but a simple gif I made a few years ago. I was asked by to update it recently, and I gladly took the chance to run it forward to the present day. So here are the direct links to a few versions updated in December 2020:

• Dark background + fast (as below)
• Light background + fast
• Dark background + slow
• Light background + slow

The following embedded version is compressed and therefore not necessarily the best one to download:

Let me know on twitter if you would like any custom versions!

NB: All data from the NASA exoplanet archive. Where masses are unknown, they are converted from radii (e.g. in the case of many transiting planets) using Weiss+Marcy mass-radius relations. While Ceres and Pluto both appear and dissappear from this plot as their planetary status became updated, disproved former exoplanets are not included in this way.