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.