Strange and Exocomet Transits

What are we talking about in this group?

Answer: Anything which deviates from the Mandel & Agol transit fit
E.g. Rings, moons, exocomets, asteroids, meteoroids, anything else

How can PLATO’s age-determinations help? Does the occurrence rate of strange transits depend on age?

Carole, Dimitri, Isabel, Babatunde, Matthew (B)

  • Age is very important for formation and evolution pathways.
  • Young systems are expected to have chaotic dynamic interactions, which might lead to more strange transits, due to disruption, so getting accurate ages for these very important.
  • From asteroseismology expect age determinations with 10% error when on the main sequence. Fraction of core H from asteroseismology is probably the sharpest tool for age determinations on main sequence.
  • Isochrones can also help; fantastic if you have a cluster etc.
  • However, young and Pre-Main-sequence ages are considerably less well defined (problems with models, understanding of physics).
  • To solve this, it would be beneficial to get asteroseismic and planetary scientists more linked up. Bill Chaplin is leading one of the asteroseismology WPs.

Other issues:

  • Integrating sun’s trajectory can only trace 10-100Myr before error ellipse gets too big to be meaningful. Generally true for all stars. (moving groups are all less than 1Gyr old)

When do planetary ring systems form? Interesting to see as a fn of stellar age. In Saturnian system there is debate about timescale for replenishment

Considered to be outcomes of impacts between moons

Can form at any time

Smaller moons are forming from these rings, which later can collide again etc.

(see Charnoz’s papers about solar System moons and the general theory)

Limiting factor to detecting exorings – distance from the star (Hill radius effect)

Detection challenge – how do we detect

Similarly for exomoons = PLATO could be the first system to find them

We really don’t know at which age exomoons form

They can form together with the planet

They can form later from rings

They can be captured at any time

A significant question is that a detectable moon must

Be formed, and

Be on enough stable orbit, and

Be detectable (large enough etc.)

Unless these three requirements fulfilled together, there will be always null detections

  • Planetary evaporation along the giant branch phases Eva Villaver (Issy’s supervisory) has studied
  • WDs are v efficient ablators due to their SED

Isochrones can also help; fantastic if you have a cluster etc. break down at young ages – need to develop models of young and pre-MS stars

The physics for these models is possibly not good enough yet.

Konstanze Zwintz was emphasizing this at Stars and their variability observed from space

We know for WD systems accretion of disrupted rocky body material diminishes over 8Gyr of WD cooling

There will be correlations if we see the same material in consecutive transits, but it will be very difficult to recognise that they belong to the same overdensity if its inner structure (and the light curve) varies a lot.

Dividing line been asteroids and comets is fuzzy

exo-Omuaumuas – ExSSiv predictions lots of these (one of the instrument talks)

Free-floating planets – can detect giant planets, but need to be in Solar system to see Omuamua

Incidence of fly-by and Galactic tides doesn’t change much until within 1 -2 kpc of the Galactic bulge.

Planetary systems in the bulge are v interesting… (Sag window? Baade window? – precise photometry of that crowded field can be a real nightmare…) 

Lasko sees comet fragments come close to the star every 3 days

The more massive the star, the more likely comet breakup is. And massive stars live shorter

Issue of how well asteroseismology works for early type stars (O,B.A stars)

Most planet-hunting missions targeted to late type stars

Luca Fossati interested in planet searches

There are polluted wds whose progenitors are more massive that the highest mass known exoplanet host stars (wd progenitor w/ 5 solar mass progenitor)

Protoplanetary discs are short-lived for high mass stars

What tools for fitting exocomet lightcurves exist and which tools could be adapted?

Matt K, Matteo, Grant, Sebastian, Gyula, Ed….


1D fast fitting tool (back in 2012):…545L…5B/abstract

2D variant (solid optically-thick body + optically thin tail) also available – developed in collaboration with Ernst de Mooij but never published.


Can we revive the publication of these 2D models? What more work is needed to get there?


Q: can we derive an “informed” set of parameters from physically-based models 

(e.g.…596A..32V/abstract) and build a fast analytical fitting tool from those?


Beta Pic exocomet fit of TESS detection:


Where to calculate simulations (what kind of central stars?)


 P1 sample – F,G,K down to 11 mag  

 P5 sample – down to 13 mag 

 P4 sample – M dwarfs down to 16 mag

Can/should we characterise the distribution of exocomet transit depths?

In principle yes given enough data. Should also be possible to make some extrapolation to depths relevant for Earth 2.0 detection (i.e. are exocomets at 1au a potential false positive?).

Should we expect any more “strange” transits from the TESS mission? What would N detections tell us about various scenarios?

How about WHeeler & Kipping Weird Objects of Interest?


Matt K wrote a Jupyter Notebook on these 14 odd transits:


Yes, but how many? There are not many exocomets seen in Kepler data. At first instance one might think that those seen are due to unexplainable systematics effects. HOWEVER, since they are all seen around the same type of objects, this speaks in favour of astrophysical origin.

Do we need systematic searches? for what?

Yes – we want to find anything that’s NOT a planet.

And for things which are ablating planets or exocomets.

Combined with a pipeline to look for single-transit planets – one which finds ALL single-transiting objects.

Can we look in the PLATO rubbish bins? Or indeed in rubbish bins from the past. How about in CoRoT


How do you remove stellar activity/variability to find the small interesting transits?

An important issue is disassociation of stellar activity artefacts from strange transits, this is a particular issue for non-periodic phenomena – like completely disrupting material


Could looking for non-periodic things and asymmetries be a good way to dissociate?


Different wavelengths (e.g. red-blue cameras on PLATO)

Better variability models are needed, especially for young stars

Follow other types of stars, not just sun-like stars – find stellar cycles

Simultaneous RV and photometric searches

What could the red and blue PLATO cameras tell us about the properties of the transiting material?

Particle size? 


Maybe can remove stellar activity in young stars to reveal grey weird transiters.


Needed to do relevant simulations first, assuming realistic composition (de we have good models for dust structures of a comet?); assuming realistic scattering functions (with angular and wavelength dependence), simulating the case and then: we will see!


Could (should?) be more in the full-frame images

Is it worth constructing a lightcurve zoo for easy reference? Or similarly for variability…

Yes, useful for eyeballing and we would need training data for machine learning.

What is the “wishlist”?

Perhaps useful for TLS-like searches where the template is used to scan the dataset.

Also very useful for human light-curve eyeballing and new people to the community


Matthew Battley
Matteo Brogi
Ed Bryant
Babatunde Akinsanmi
Dimitri Veras
Sebastian Zieba
Carole Haswell
Isabel Rebollido
Pam Rowden
Gyula M. Szabó
Paul A. Wilson