How star spots trick astronomers: a chat with Dr Mayuko Mori

Welcome to Growing in Worlds, the podcast where we explore the science of exoplanet through conversation with early-career researchers on the front line of discovery. I'm Lorenzo.

And I'm Jenny. In each episode, we meet a guest who takes us behind the science of planets, stars, and the search for life in the universe.

Today we turn our attention again to the stars that host planets, and more specifically to the dark, ever-changing features on the surfaces, star spots. These stellar blemishes may look small, but they can have a big impact on how we interpret exoplanet data.

To help us understand how studying these star spots can reveal more about distant worlds, we're joined by Dr Mayuko Mori from the Astrobiology Centre in Tokyo. Mayuko, welcome to Growing Worlds!

Hi, thank you so much for inviting me.

I'm delighted to have you, really.

Mayuko is a JSPS postdoctoral researcher fellow at the Astrobiology Center in Tokyo. She completed her PhD at the University of Tokyo, where she focused on the interplay between stellar activity and exoplanet observation, in particular how star spots affect what we see when a planet transits his hot star. So her work combines multiband photometry and spot-crossing analysis.

Thank

and stellar modeling to disentangle the light and the star from that of the planet. Something you could have observed like a planet system like K225 or TOI 3884b showing how monitoring the star spot with even a small or medium-sized telescope, we are talking about one or two meter diameters, can reveal hidden complexity of exoplanet systems. Now I also have a little surprise here for Jenny because Jenny doesn't know that, yeah, but we have a little connection here. So as our listener knows, Jenny and I first met while we were working on an observation of WOSPA 103b and this observation was during her PhD and my master thesis. And this observation was performed with a Japanese telescope in South Africa, which is the IRSF.

Hello? Ooooo Yes. Mm-hmm. Yes.

If I remember correctly, Mayuko also started her journey analyzing data from that very same telescope.

Yeah.

goodness I love that

We all started from the same kindergarten.

Yeah, yeah, and what a telescope my go did you ever go out there and use it? my goodness

That's true.

Yes, when I was a second year master student. was like the first, sorry, yeah, we used the IRSF telescope for the test follow up. When I was a master student, the test satellite started its survey and then it started from the Southern Hemisphere. So we needed Southern telescope to follow up the

Yeah, Okay.

transit of the state's target. So we had a Japanese telescope in South Africa so that's why we went there to do that.

All I remember about that telescope is how hands-on you had to be, like opening the shutters and those big clunky buttons to like turn it on and you had to like manually put in so many commands to, you know, open up the mirror and do what I was... I feel like it was almost like a trial by fire. It was good.

Yes. If

You

Yes. Yeah, even we needed to climb up the telescope to remove the stick to like move the telescope. You remember?

Yeah, yeah and then I remember at one point the telescope just got stuck as they do sometimes and we had to call an engineer because it just wouldn't move and he just like unlocked everything and then he was just moving the telescope by hand. I was terrified but yeah he was absolutely fine, he just locked it all back in place and he's like right carry on.

Dear listeners, this is what happens when observers match.

yeah this is it we forget about the task at hand and we just start talking about the telescope. What I also remember is that there is a rice cooker in the warm room of that telescope and it saved us at like

Yeah

three o'clock in the morning when we were so hungry, like we would get the rice cooker going and we could have like this lovely hot rice just to like keep us going through the night. And I was always really grateful for that. And also for some reason, maybe you can answer this, there was also an incredible sound system, like with like massive speakers and like, you know, subwoofers for, just, what? Yeah, I don't know. Maybe that was put in.

really?

like either it was taken away when you weren't there or it was put in after you went but when I was there there was this like enormous sound system and I never really understood why.

That's nice, I didn't try well.

we did when it was cloudy one night we were like hmm We can't do anything. Let's try it out. It was loud

You

But I feel like we should probably actually get back to the task at hand instead of reminiscing over observing in South Africa. So I think it's always nice to start with a personal journey. So Mayuko, why don't you tell us what drew you into astronomy in the first place and then into the world of stars and exoplanets?

Yeah, from like, since I was a high school student, I really, I was really inspired by my physics teacher in my high school. So I, yeah, I was got into physics. And then in undergraduate, I really interested in astrobiology. So I was naively, yeah, looking at the stars, I was naively thinking that maybe there are

Love that. Okay.

another life in other stars. I was thinking that it just now it's just a time before like age of discovery. So there should be plenty of life elsewhere, like other places than the solar system. Yes, that's why I really interested in astrology and try to take many biology classes and astronomy classes. Yeah, just simply I thought astrobiology is a combination between astronomy and biology. But then, yeah. But then.

Yeah. Yeah. So then how did you end up on the sunspots? Well, star spots I should say, because it's, you know, sunspots on other stars, right?

Yes. Yes, yes. For my master thesis, I was given the data of a TRAPS-1G's transmission spectrum to do the atmosphere analysis of TRAPS-1G. And TRAPS-1G is one of the potential habitable planets, and it's very interesting systems. So it has some connection between astrobiology to a TRAPS-1. But then, yeah.

Okay.

I found amazing that your dream was astrobiology and you actually end up in the astrobiology center of Japan. It's really amazing. You started with this in mind and you actually landed there.

Yeah. Yeah.

All right, that's very true.

Yeah, but then yeah, when I dealing, when I was trying analyzing TRAPPIST-1 dataset, I realized that it's like, I'd say the precision of the data is not very good as which I can do any analysis on atmosphere. But then I realized that the dataset is still useful to do the analysis on the stellar activity of TRAPPIST-1. And then I realized how the spot effect is big in the atmosphere spectrum. And I realized that we need to know still activity or star spots before the planetary atmosphere. at that time, effect of star spots was already, yeah, already, let's see, the effect of star spots was already admired, but not very popular.

Okay.

So I thought it's a good niche area of astronomy to investigate, but now it's super, super popular and important.

Mmm. Yeah. Now, but that is because you've helped make it super popular and important, you see?

Can.

yeah, one country.

Can you walk us through that? mean, you just say that it's becoming more more important. Can you explain us what the spots are, specifically for the listener who might not know, and why they are so important for the exopratic characterization?

Yes, spots is the optically dark and cool area on the stellar surface, which is created because of the stellar magnetic dynamo inside the stellar surface. So the physics mechanism to create star spots is that the convection on the stellar Inside the stellar surface, there is a convection of the hot gas. But if there is a strong magnetic field on one part of the stellar surface, that strong magnetic field can suppress the convection. And then that area becomes cooler and darker in optical. And those kind of star spots can be observed in other systems rather than only in the sun. And those star spots can create inhomogeneity. of the stellar surface. And if we don't account those stellar surface inhomogeneity, then those cooler area or those inhomogeneity can create or mimic the atmosphere signal in the transmission spectrum of the planets.

wow, so it is absolutely crucial to make sure that if there are star spots there that we know about them.

Yes. Yes, especially for small rocky planets, stellar effect is sometimes even comparable amplitude in the signal, comparable to the planetary atmosphere signals.

Okay.

So, I mean, how do you detect and measure star spots on other stars? I just, I'm so fascinated by how this is even possible because there's so few stars that we can actually like see a surface for, know, at least resolve them into not a point. how, how?

Yes.

Right. Yes, yeah, in many observations, we need to assume some models, but there are several observational things that we create, star spots create. So one is the brightness modulation, which is if we look at the stars, if we monitor the stellar brightness, then stars sometimes become a little brighter and a little fainter, which is because of the rotation of the spotted surface. If we see more spots, the star looks darker. And if there is less spots, the star looks brighter. So those kind of modulations, we can infer that there are spot covering fraction changing.

Mm. yeah.

Okay.

Okay, so it's about monitoring the stars for a long time to see sort of what the baseline is and then seeing where it dips off.

Yes. Yes, usually. Yes, usually the stellar rotation period is around several days to several tens of days. So we need to monitor the star for a longer period to recover the modulations.

Yeah.

Okay, so can you figure out is the stellar rotation something you kind of measure? So you sort of know, okay, this star takes like 10 days or is it something that you get from like stellar models? So you sort of say, well, this star is, I don't know, 15 solar masses, therefore it has a rotation period of X.

Usually we need to measure the brightness, we need to infer the period from the brightness change. the star, if, yeah, sometimes if there is no spots, it's very difficult to measure the rate. If we do the high resolution spectroscopy of the stars from the broadening of the stellar spectra, we can also infer the stellar rotation period.

Okay. Mmm.

Okay, but if you are inferring it from the modulation of the brightness, how does it compare to the actual modulation of the brightness, which is called light curve intensity in exoplanet? I mean, how can you distinguish between the two of them? Is there a shape difference for something?

Hey.

Stop. Yes, so usually the rotational modulation is more like a sine curve like of fluctuations and also it continuously goes up and down but for transit, transit is only happening when the planet is passing in front of the star so it's usually that the light brightness is stable and only during the transit the brightness is decreasing and coming back to the normal.

Mm.

So because of the shape and also the time scale differences, we can differentiate those two.

So especially if the trans and exoplanet, because if you're in particular interested in sort of ones that are in the habitable zone, I guess they'll probably have a longer orbital periods than it takes for the star to rotate. So then that can help you distinguish, I guess as well.

Yeah. Yes.

But sometimes, stereo rotation also makes it difficult to detect planetary transit signals. especially for young active systems, the stereo rotation is very large, like 1 % or 2%, even 10 % difference change in the brightness. In that case, the small change of a small dip because of the transit is like cancel out and it's very difficult to detect.

wow.

Okay. And you can correct those light curves and recover the transit of the planet.

Yeah, if you use a very sophisticated method to detect those small, like, let's say, dip, and correcting for the large modulation, we can sometimes be able to detect the transit signal.

No, go on, go.

have difficult question now. And what if the planet is transiting on a star spot?

Yeah, that is called spot crossing. So if the planet passing in front of the star, then that means the... If the planet is passing in front of the star spot, that means the planet is crosses a relatively darker area on the stellar surface. And that creates like the star looks a little... brighter during the spot crossing, it creates the bump in the transit light. So it's opposite transit during the transit type of weird signal, like W-shaped signal during the transit. So from.

No.

Okay.

Yeah. wow yeah, I never thought about that. Yeah, so you have your transit dip and then there's a little bump in the middle when it's like blocking out the dark sunspot. Starspot. I gotta stop saying sunspot. I'm just so used to thinking about them with the sun.

Yeah. Yes.

Hahaha

Hehehe, so cool.

You

Yes, so from those spot crossing signals, we can infer the stellar spot size or spot temperature or even spot location. spot crossing is actually a big noise for transit observations, actually. But also, that spot has many information about stellar surface intermodunity. Especially from rotational modulation, it is very difficult to infer the spot like exact spot location, but for drotting, yeah, we can't know that.

right but with the crossing you know. So are there certain stars that have more star spots or less star spots? And then how does that connect in terms of the systems where we're more likely to find small rocky worlds in the habitable zone?

Sorry, so you mean which stereotype has more star spots?

Yeah, and then are they, is there any kind of connection with then those stars and having like small rocky worlds in the habitable zone?

So usually star spots appear more on a magnetically active star. usually young systems, especially less than like one giga year stars, is very active. So it tends to show big star spots or more star spots on the star surface. Yeah. So

Okay.

I'm not very sure that such young systems are actually habitable or not, but it's very important to know those young systems, to know how the planetary system evolves or how even the habitability changing. yeah, searching for those young

Mmm. Yeah, so the stars sort of calm down as they age.

Yes, usually yes. For red dwarfs, like with the temperature less than 3000 Kelvin or 4000 Kelvin, even for older age stars still have magnetic activity. So for red dwarfs, the planetary systems around red dwarfs, it's still complicated for even older. So as traps one. Traps one is known to be old.

Mmm.

already old, but it still shows strong magnetic activity and star sports are very problematic for atmosphere research on transport bodies.

Okay, so, those red dwarf stars are the most common stars in our galaxy, so it's unfortunate that the most common stars are the most troublesome when it comes to star spots.

Right, that's right there.

Of course, of course. Nothing is never easy.

Yeah! I've lost track of which questions we're on now as well because we've just been flying through. It's okay, we'll cut this. Lorenzen will cut this little bit out. I'm so sorry, Michael.

Me too. Yeah, yeah, will, I will cut this, but we were just caught in the conversation. We were like, okay, what, what, what should we ask actually? So, back on track. I think we are here.

Hehehehe Yeah.

What have we covered and what haven't we covered? Yeah. Quite okay, yeah, yeah, I see, I see. Okay, you carry on then.

Okay, so, okay, so.

So, red dwarfs are complicated. It's difficult. Magnetic field is affecting how the planet forms, how they evolve, but also the star spots that messed up the way we measure them. So, focusing on the star activity part, it seems to me that even small changes in the stars can lead to big differences in how we measure the signal. Therefore, guess even how we interpret the signal, right? what are the limits today? How precisely we can model or correct for this stellar effect in order to get back to the actual information of the planets?

How... how... sorry.

how well can we detrain the destabilized activity out of the planetary transit.

Yes, so... Sorry, yeah, though.

No, it's okay. It's alright. We can cut out the breaks and the pauses, so don't worry.

Hey.

We can even skip a question if you think it's... You don't have the answer. Okay.

It's okay. yeah, for to... Usually we use... Usually we recover the planetary signals and the stellar surface signal simultaneously in the... to recover the planetary signals from the transmission spectrum. But we need to rely on the many assumptions on stellar state, like spot temperature or spot size. There are a lot of assumptions. This is current measure. yeah, in the future, we need to know more small differences in the transmission, in the transit depth according

Mm.

among the wavelength stream. So to reach the precision we need, we actually need to know the We actually need to know the real physics inside a star to create the star spot or stellar surface in homogeneity. So yeah, we need to know the stellar surface itself from the observational fact and analysis.

So if you think about the next decade, let's say, the next 10 years, what we will need to improve on this side?

Yeah. So first we need to observe more star spot using like James Webb or a very sophisticated instrument to derive the spectrum of star spot themselves. So now we assume star spot is just like 100 Kelvin or several hundred Kelvin cooler than the stellar surface. And we use a stellar spectrum models.

Okay.

But we are not very sure those models are completely trace the actual spectrum model of the star spots. So maybe star spot crossing is a very good phenomena to observe with James Webb. So if we can get the actual information of the star spot spectrum from James Webb data, then we can compare the theoretical prediction. And also we can infer the actual physical state like metallicity or surface gravity of a star spot. And those information are very important to input to the atmospheric recovery.

how would you use JWST to do that? Because I guess you can't predict when there's gonna be a star spot or a star spot crossing. And so I know that like, it's so difficult to get JWST time. Is it a case of you just have to sort of pick, I don't know, 20 stars, observe them and hope, just that like kind of look at the right field and have the right number of stars in it and just hope?

Yes.

that there is a star sport crossing, is that the way forward?

Yeah, it's very difficult to predict when the spot crossing is happening. One interesting system is TOI-3AA4 system, which also I studied before. So it always shows spot crossing signals, so that it's very useful and very unique.

Okay.

Huh.

So is that because it's got lots of planets in that system and so there's like more chances for crossings or is it just they orbit really quickly or?

or just a lot of spots.

Yeah, T-3A4 is just a very unique system, which always shows spot crossing, and that spot crossing is happening in the exact same, almost same time in the transit. That means, first, when we first find those effects, many astronomers were thinking about what it is. And then, current prediction is that

Mm.

The system is a say polar on system. We are looking the star from the pole of the star and planet is orbiting in polar on orbit. Like pole on orbit. Yeah, planet is planetary orbit is

wow. Yeah. It's like tilted 90 degrees.

90 degrees tilted to the stellar inclination. And also there is a big polar spot on the stellar surface. Then always when the planet crosses near the pole, the planet can cross the spot on the stellar pole. So it's very unique.

Yeah. wow! Yeah.

Wow.

Wow, so there's like a permanent star spot at the pole of the star. Is that common for all stars or does it just seem to be this particular star?

No, think... Yes, I think this is only one case now we know so far.

I wonder why that is. I guess it's just permanent disruption of the magnetic field caused by something like, well, the convection, right? It's the convection stops the hot gas rising and then that's why you get a cold spot and that's caused by the magnetic field. So it's something like, is it because it's at the poles? Cause I know that, so they just got the first view of the sun south pole, right? With the European Space Agency.

Yeah. Yes.

I forget the name of it now it's gone out, but we've just had to look at the Sun South Pole and, and that the magnetic field there is super tangled, right? It's like North and South magnetic field lines all mixed up and tangled. So yeah, maybe at a different, a different point in the stellar cycle, you, can have these like permanent star spots at the poles for, for, you know, for a number of years. I've never heard of that before. That's so exciting.

Hmm. Yes. There are still very limited observational facts, but some predict that the polar spot might be common among active systems, active stars. just guess, or even not Yang, but active emdwarfs.

you

So like young stars. Okay, yeah, yeah, I'm with you.

Yeah, TOS-3A4 is also a red dwarf. But it's just because we don't have such full orbit planet. We are not very lucky to find those systems. So that's why it's very difficult to find the similar system. But we don't know. Yeah, only the one.

Yeah. Yes, in order to repeat, yeah. And then of course there's question, how on earth does a planet get into a polar orbit like that? There's some serious dynamics going on there, right?

Something really weird, yeah.

Yeah, to make the planetary orbit to be a perfect, like the high obliquity, the high spin orbit axis, high spin orbit angle, we need some kind of perturbation from outer planet or other stars or companions. We need some other stellar bodies.

So yeah, no, carry on. you

For TWSRF system, actually, we don't know there is. We didn't find out a planet or any other companions. So it's still mystery. Yeah.

It's a mystery.

Mystery about sky. Yeah, it's amazing

So do you think, aside from like JWST staring at this particular system, there like other upcoming missions that will help enhance our understanding of star spots that are coming up in the future? Maybe like Plato or Ariel or anything else? Are there any other kind of observatories?

Hmm.

Yes, Ariel is also doing the similar thing, like transmission spectroscopy of many, many planetary systems. So maybe we should look at spot-crossing signals for some systems. Then, yeah, we can collect those information to derive the spectroscopic information of the star spot. And also not only for still space-based missions, but also ground-based telescopes, even our... Yeah, only for monitoring or spot crossing. Photometry though. Yeah, from those spot crossing photometry or monitoring using the small-sized or middle-sized telescopes, we can get the general information of the star spots, like usual temperature or usual size of the star spots or like the polar spots are common or not, those kind of things.

you can do it from the ground. Okay. Yeah.

We even don't know those general information of the star sports other than sun sports. So yeah, we should correct the information.

Okay. So what information can you get from photometry and what information can you get from spectroscopy about the star spots?

Usually spectroscopy has more information, but like what molecules are inside a star spot or what metallicity or those kind of information, temperature. But even from photometry, if we have multiband dataset, we can infer the spot temperature. Spot temperature difference from the stellar surface temperature and also

Yeah. Wow. Yeah.

Usually for photometry, we have more time to observations, so we have more long period data set. Then if we monitor data for a long time, maybe we can get activity changing. Sometimes the spot or there is more spot on the set of surface and sometimes are not similar to solar cycle. So those kind of activity cycle or yeah, those kind of information can be.

you Yeah.

accessible from the ground-based photometry.

And can you tell us how do you get temperature out of the photometry and how do you get like temperature out of the spectroscopy and like the metallicity out of the spectroscopy? I'd love to know.

Hehe. Okay, yeah, from the photometry is that the, how to say the. Difference of the spot contrast, spot brightness compared to the stellar surface is different according to the wavelength. And it is coming from the difference in the blackbody emission differences from the stellar surface and the photosphere. In some wavelength, especially for blue wavelengths, blue band, the spot looks

Okay, yeah. Okay. Mm.

darker compared to the stellar surface. spot emission is smaller compared to the emission from the stellar surface. But in red bands, the spot doesn't look such dark. So then the spot contrast is close to 1. So then, yes, that's kind of like, especially for the rotational modulation, the modulation amplitude is connected to it, that such spot contrast. then usually in blue band spot, rotational modulation is much bigger. And in red band, it's smaller. So from those amplitude difference, we can infer the spot contrast, and then we can infer the spot temperature. And for spectroscopy, yeah, I'm not very familiar with spectroscopic observations, but we have a stellar spectrum.

Yeah.

models. So from comparing models to the observed spectrum, the depth of each absorption lines or emission lines of which molecules or elements we can derive the spot temperature or metallicity.

Okay. Isn't that amazing? It's just incredible that you can figure out like the temperature of star spots from, from, know, taking pictures at different wavelengths and just seeing how the light is relatively changing. And that's brilliant. I love space.

It's kind of magic how we can infer all this information about objects that are light years away from us, just from the picture that we take. I love science. But I'm not the only one loving science. I know that there are a lot of people out there, not only us on the podcast, but of course also our listeners.

Yeah. Yeah. Yeah.

And I'm wondering, you say that you can do this follow-up work with small and medium class telescopes like one or two meter class telescopes? I think he... Yes! Who doesn't have a one meter telescope in their garden, right? But assuming I have one and I want to contribute, how will I do it? How can I contribute to this kind of follow-up?

I love that small is one meter.

I'm

Yes!

Yeah, actually, I'm now very interested in gathering those amateur telescope things to do the spot observations. One is monitoring observations, and the other is spot-crossing transit observations. I think even 50-centimeter, 60-centimeter telescopes can do those kinds of observations if we have a good setup. So recently, there is exoclock work. run by ARIEL team. X-CLOCK is a project that gather all the professional or amateur astronomers to collect the transit observations of the planetary systems. And then from those transit data sets, they're updating the transit timing predictions. But at the same time, I think also those kind of collected transit data is useful to analyze if there is spot crossing observable or not. So already there is many amateur data sets about the ARIA targets. And I'm now very interested in looking at exoclock data set to do the analysis of spot crossings. So yeah, if you have small data set,

Mmm. I love that. yeah, people can get involved and give you the information that you need for like when these spot crossings are happening and just general baseline monitoring and that's amazing. I love that.

And joining XsoClock, dear listeners, is really easy. Just go to the website XsoClock and there you'll find all the instructions on how you can actually contribute uploading your observation and help Mayuko then.

Hehe.

So I think before we wrap up, it is time to finish on our signature question, our final question that we ask everyone, which is if you had to choose one celestial body, a star, a planet or something else entirely to represent you, what would it be and why?

Yeah, it's really good question, but maybe.

It is a difficult question, it is. It's a fun one though, we think it's fun anyway.

Yeah, I thought maybe K-25 star. It is the target of my PhD thesis. And yeah, it's around 600 million years old. So it's quite, it's kind of young, but not very young system currently, but it's still active. So maybe similar to me.

Okay. Yeah, like that.

And also, yeah, it was very popular before, but now it's not super popular, not as famous as some even younger systems being discovered recently. But yeah, I find these traditional age systems, it's very fascinating. Yeah, even K225 is very fascinating because they can tell us how the planetary systems evolve. And it has a lot of information about.

Mmm.

vital systems and also still activity. So yeah, in a way, yeah, I like the thing I have.

Yeah. I think that's good, because yeah, because you are certainly fascinating and you definitely have a lot of information about stars and planetary systems as well, so I think that's a good choice.

Thank Yeah, I hope to be still active and have a potential to, yeah.

Mmm. Absolutely, I think so.

Thank you. Thank you, Faiyuku, for showing us how observing these subtles flickering and shadows on star can reveal that much about them and about the worlds that orbit in them. Thank you very much for being with us.

Thank you so much.

Thank you to everyone listening. Join us next time on Growing Worlds as we explore another corner of the universe and the people bringing it closer.