Tag: transit

Visionaries Old And New

On By rolcottIn Astronomy, UncategorizedLeave a comment

Cathleen’s back at the mic. “Let’s have a round of applause for Maria, Jeremy, Madison and C‑J. Thank you all. We have a few minutes left for questions… Paul, you’re first.”

“Thanks, Cathleen. A comment, not a question. As you know, archeoastronomy is my specialty so I applaud Jeremy’s advocacy for the field. I agree with his notion that the Colorado Plateau’s dry, thin air generally lets us see more stars than sea‑level Greeks do. When I go to a good dark sky site, it can be difficult to see the main stars that define a constellation because of all the background dimmer stars. However, I don’t think that additional stars would change the pictures we project into the sky. Most constellations are outlined from only the brightest stars up there. Dimmer stars may confuse the issue, but I very much doubt they would have altered the makeup of the constellations a culture defines. Each culture uses their own myths and history when finding figures among the stars.”

“Thanks for the confirmation from personal experience, Paul. Yes, Sy?”

“Another comment not a question. I’m struck by how Maria’s Doppler technique and Jeremy’s Astrometry complement each other Think of a distant stellar system like a spinning plate balanced on a stick. Doppler can tell you how long the stick is. Astrometry can tell you how wide the plate is. Both can tell you how fast it’s spinning. The strongest Doppler signal comes from systems that are edge‑on to us. The strongest Astrometry signal comes from systems we see face‑on. Those are the extreme cases, of course. Most systems are be at some in‑between angle and give us intermediate signals.”

“That’s a useful classification, Sy. Madison’s and C‑J’s transit technique also fits the edge‑on category. Jim, I can see you’re about to bust. What do you have to tell us about?”

“How about a technique that lets you characterize exoplanets inside a galaxy we see as only a blurry blob? This paper I just read blew me away.”

“Go ahead, you have the floor.”

“Great. Does everyone know about Earendel?” <blank looks from half the audience, mutters about ‘Lord Of The Rings?’ from several> “OK, quick refresher. Earendel is the name astronomers gave to the farthest individual star we’ve ever discovered. It’s either 13 or 28 billion lightyears away, depending on how you define distance. We only spotted it because of an incredible coincidence — the star happens to be passing through an extremely small region of space where light in our general direction is concentrated thousands‑fold into a beam towards us. Earendel may be embedded in a galaxy, but the amplification region is so narrow we can’t see stars that might be right next to it.”

<Feder’s voice> “Ya gonna tell us what makes the region?”

“Only very generally, because it’s complicated. You know what a magnifying lens does in sunlight.”

“Sure. I’ve burnt ants that way.”

“… Right. So what you did was take all the light energy hitting the entire surface of your lens and concentrate it on a miniscule spot. The concentration factor was controlled by the Sun‑to‑lens‑to‑spot distances and the surface area of the lens. Now bring that picture up to cosmological distances. The lens is the combined gravitational field of an entire galaxy cluster, billions of lightyears away from us, focusing light from Earendel’s galaxy billions of lightyears farther away. Really small spots at both ends of the light path and that’s what isolated that star.”

“That’s what got you excited?”

“That’s the start of it. This new paper goes in the other direction. The scientists used brilliant X‑ray light from an extremely distant quasar to probe for exoplanets inside a galaxy’s gravitational lens. Like one of your ants analyzing sunlight’s glare to assess dust flecks on your lens. Or at least their averaged properties. A lens integrates all the light hitting it so your ant can’t see individual grains. What it can do, though, is estimate numbers and size ranges. This paper suggests the lensing galaxy is cluttered with 2000 free‑floating planets per main‑sequence star — stars too far for us to see.”

~~ Rich Olcott

  • Thanks to Dave Martinez and Dr Ka Chun Yu for their informative comments.

To See Beneath The Starlight

On By rolcottIn Astronomy, James Webb Space Telescope, UncategorizedLeave a comment

C‑J casts an image to Al’s video screen. “This is new news, just came out a couple of weeks ago. It’s the lead figure from NASA’s announcement of JWST’s first exoplanet examination. We’re picked this study because the scientists used the transit technique. I’ve added the orange stuff so we can make a point. Each blue dot is one measurement from JWST’s Near‑Infrared Spectrograph while it looked at a star named LHS 457. Even though the telescope is outside Earth’s atmosphere and operating at frigid temperatures, you can see that the numbers scatter. Surely the star’s light isn’t changing that quickly – the dots are about 9 seconds apart – the spread has to come from noise in JWST’s electronics.”

Adapted from image by NASA Credit: NASA, ESA, CSA, L. Hustak (STScI).

“We’re just partway through our statistics class but we know to expect 95% of noise to be within 2 standard deviations either way of the average. With about 400 dots per hour, C‑J drew his lines to put about 10 dots per hour each above and below.”

“Right, Madison, and the point we want to make is how small that range is. Only about 0.04% difference. That’s like one drop in a 2500‑drop titration. Professor Kim’s samples in our Chem lab generally take around 20 milliliters which is about 400 drops.”

“So anyway, look at that dip in the light curve. That’s way out of the noise range. The starlight really did dim, even though it wasn’t by much.”

“By the way, NASA’s press release is a little misleading and in fact missed the point of the research. JWST didn’t find this exoplanet, the TESS satellite system did. JWST looked where TESS said to and yup, there it was. This report was really about what JWST could tell us about the exoplanet’s atmosphere.”

“There’s a bunch of possibilities that the researchers can now eliminate. C‑J, please cast the next slide to the screen. We need to be clear, this isn’t the spectrum that JWST recorded during a transit.”

Adapted from image by NASA (Credit: NASA, ESA, CSA, L. Hustak (STScI)), and Figure 2 in Lustig-Yeager, et al.

“No, that would have been simply the star’s light after some of it was filtered through the planet’s atmosphere. The researchers used a lot of computer time to subtract out the right amount of the star’s own spectrum. This is what’s left — their estimate of the spectrum of the planet’s atmosphere if it has one. I added the orange error bar on each point and for the sake of comparisons I traced in that dotted curve marked ‘Metallicity‘ from the scientists’ paper. The other lines are models for four possible atmospheres.”

“Why orange again? And why are the bars longer to the right of that gap?”

“I like orange. I had to trace the bars for this slide because NASA’s diagram used dark grey that doesn’t show up very well. The dots in the wavelength range beyond 3.8 microns are from a noisier sensor. Professor O’Meara, we need some help here. What’s metallicity and why did the paper’s authors think it’s important?”

“We haven’t touched on that topic in class yet. ‘Metallicity’ is the fraction of a star’s material made up of atoms heavier than hydrogen and helium. A star could have high metallicity either because it was born in a dust cloud loaded with carbons and oxygens, or maybe it’s old and has generated them from its own nuclear reactions. Either way, a planet in a high‑metallicity environment could have an atmosphere packed with molecules like O2, H2O, CH4 and CO2. That doesn’t seem to be the case here, does it?”

“No, ma’am. The measured points don’t have this model’s peaks or valleys. Considering the error bars, the transmission spectrum is pretty much flat. Most of the researchers’ other models also predict peaks that aren’t there. The best models are a tight cloud deck like Venus or Titan, or thin and mostly CO2 like Mars, or no atmosphere at all.”

“Even a null curve tells us more than we knew.”

~~ Rich Olcott

Significant Twinkles

On By rolcottIn Astronomy, UncategorizedLeave a comment

Cathleen’s got a bit of fire in her eye. “Good exposition, Jeremy, but only just barely on‑assignment. You squeezed in your exoplanet search material at the very end. <sigh> Okay, for our next presentation we have two of our freshmen, Madison and C‑J.”

“Hello, everybody, I’m Madison. I fell in love with Science while watching Nova and Star Trek with my family. Doctor O’Meara’s Astronomy class is my first step into the real thing. C‑J?”

“Hi, I’m C‑J, like she said. What started me on Astronomy was just looking at the night sky. My family’s ranch is officially in dark sky country, but really it’s so not dark. Jeremy’s also from the High Plateau and we got to talking. We see a gazillion stars up there, probably more stars than the Greeks did because they were looking up through humid sea-level air. On a still night our dry air’s so clear you can read by the light of those stars. I want to know what’s up there.”

“Me, too, but I’m even more interested in who‘s up there living on some exoplanet somewhere. How do we find them? We’ve just heard about spectroscopy and astrometry. C‑J and I will be talking about photometry, measuring the total light from something. You can use it even with light sources that are too dim to pick out a spectrum. Photometry is especially useful for finding transits.”

“A transit is basically an eclipse, an exoplanet getting between us and its star—”

“Like the one we had in 2017. It was so awesome when that happened. All the bird and bug noises hushed and the corona showed all around where the Sun was hiding. I was only 12 then but it changed my Universe when they showed us on TV how the Moon is exactly the right size and distance to cover the Sun.”

“Incredible coincidence, right? Almost exactly 100% occultation. If the Moon were much bigger or closer to us we’d never see the corona’s complicated structure. We wouldn’t have that evidence and we’d know so much less about how the Sun works. But even with JWST technology we can’t get near that much detail from other stars.”

“Think of trying to read a blog post on your computer, but your only tool is a light meter that gives you one number for the whole screen. Our nearest star, Alpha Centauri, is 20% larger than our Sun but it’s 4.3 lightyears away. I worked out that at that distance its image would be about 8½ milliarcseconds across. C‑J found that JWST’s cameras can’t resolve details any finer than 8 times that. All we can see of that star or any star is the light the whole system gives off.”

“So here’s where we’re going. We can’t see exoplanets because they’re way too small and too far away, but if an exoplanet transits a star we’re studying, it’ll block some of the light. The question is, how much, and the answer is, not very. Exoplanets block starlight according to their silhouette area. Jupiter’s diameter is about a tenth the Sun’s so it’s area is 1% of the Sun’s. When Jupiter transits the Sun‑‑‑”

“From the viewpoint of some other solar system, of course—”

“Doesn’t matter. Jupiter could get in between the Sun and Saturn; the arithmetic works out the same. The maximum fraction of light Jupiter could block would be its area against the Sun’s area and that’s still 1%.”

“Well, it does matter, because of perspective. If size was the only variable, the Moon is so much smaller than the Sun we’d never see a total eclipse. The star‑planet distance has to be much smaller than the star‑us distance, okay?”

“Alright, but that’s always the way with exoplanets. Even with a big planet and a small star, we don’t expect to measure more than a few percent change. You need really good photometry to even detect that.”

“And really good conditions. Everyone knows how atmospheric turbulence makes star images twinkle—”

“Can’t get 1% accuracy on an image that’s flickering by 50%—”

“And that’s why we had to get stable observatories outside the atmosphere before we could find exoplanets photometrically.”

~~ Rich Olcott

Teena Meets The Eclipses

On By rolcottIn Astronomy: Planetary Science, Astronomy: Stellar Science, UncategorizedLeave a comment

“Don’t look up until it suddenly gets really dark, Teena.  I’ll tell you when it’s time.”

“OK, Uncle Sy.  Oooo, look at the house where our tree makes a shadow!  It’s all over crescents!”

“Yep, wherever leaves overlap to make a pinhole, it’s like the one we made in our cardboard.  See, those crescents are just like the one our pinhole beams onto the sidewalk.”

“Yeah.  ‘Cause it’s the same Sun, right?”

“Sure is.”

“Are other little kids seeing the eclipse all over the world?  They’ve got the same Sun, too.”

“No, just the ones who happen to be on the shadow stripe that the Moon paints on the Earth.”

“How many kids is that?”

“Hard to tell.  Some families live where the shadow passes through, some families travel to be there, lots of other families just stay where they are.  No-one knows how many of each.  But we can make some not-very-good guesses.”

“The crescent’s going so slow.  Let’s make guesses while we’re waiting.”

“OK.  Let’s start by imagining that all the world’s people are spread evenly over the land and sea.”

“Even on the ocean?  Like everyone has a little boat?”

“Yep, and sleds or whatever on polar ice, people everywhere.  In our city there are eight blocks to a mile, so if we spread out the people there’d be one person every other block.”

“Every other block.  Like just on the black squares on our checker board.”

“Uh-huh.  The Moon’s shadow today will be a circle about 80 miles across and it’ll travel about 2500 miles across the whole country.  The stripe it paints would cover about 6½ million spread-out people.  Maybe 10 million if you count the people in little boats, ’cause the eclipse starts and ends over the ocean.”Local eclipses

“Lots of people.”

“Yes, but only about one person out of every thousand people in the world.”

“We’re pretty lucky then, huh?”

“Oh, yeah.”

“Are there eclipses on other planets?”

“Of a sort, but only for planets that have a moon.  Poor Mercury and Venus don’t have moons so they never see an eclipse.”

“Aww. … Wait — you said ‘of a sort.’  Are there different kinds of eclipses?”

“You’re very alert this morning.  And yes, there are.  Two that get the publicity and two that we never see on Earth.  It has to do with perspective.”

“Per … perspec…?”

“Perspective.  The word originally meant very careful looking but it’s come to be about how things look from a particular point of view.  See that tree across the street?”

“Yeah.”

“Think your hand is bigger than the tree?”

“Of course not.  I climb that tree.”

“OK, put your hand between your eyes and the tree.”

“Oh!  My hand covers the whole tree!”

“Yup.  Nearer things look big and farther things look small.  That’s perspective.  Eclipses are all about perspective.”How big is the Sun

“How come?”

“The perspective principle works in the Solar System, too.  If you were to travel from Earth to Mars to Jupiter and so on, the Sun would look smaller at each planet.”

“Like the far-away trees look smaller than the close trees.  But what does that have to do with eclipses?”

“A planet gets an eclipse when one of its moons comes between it and the Sun.  That’s what’s happening right now here.  Our Moon is moving between us and the Sun and blocking its light.”

“But I don’t see the Moon, just the carved-out piece.”

“That’s because we’re looking at the unlit side of the Moon.  It’s so dim compared to the rest of the sky.  Anyway, the Moon’s width we see is just about the same as the Sun’s width.  The moons on the other planets don’t match up that well.  On Mars, for instance, its moon Phobos appears less than half the width of the Sun even though the Sun appears only 2/3 as wide as we see it.  Phobos can never cover the Sun entirely, so no true eclipse, just a transit.”

“Can the planet’s moon be bigger?”

“Sure.  On Jupiter, Europa’s width completely blocks out the Sun.  That’s called an occultation.  You can look up now.  Jupiter people can never see that corona.”

“Oooooo, so pretty.  We’re lucky, aren’t we?”

“In more ways than you know, sweetie.”

~~ Rich Olcott

The Luck o’ The (insert nationality here)

On By rolcottIn Astronomy: Stellar Science, Astronomy: Techniques, UncategorizedLeave a comment

“Afternoon, Al.  What’s the ruckus in the back room?”

“Afternoon, Sy.  That’s the Astronomy crew and their weekly post-seminar coffee-and-critique session.  This time, though, they brought their own beer.  You know I don’t have a beer license, just coffee, right?  Could you go over there and tell ’em to keep it covered so I don’t get busted?”

“Sure, Al.  … Afternoon, folks.  What’s all the happy?”

“Hey, Sy, welcome to the party.  Trappist beer, straight from Belgium!”

“Don’t mind if I do, Cathleen, but Al sure would like for you to put that carton under the table.  Makes him nervous.”

“Sure, no problem.”

“Thanks.  I gather your seminar was about the new seven-planet system.  How in the world do the Trappists connect to that story?”

“Patriotism.  The find was announced by a team from Belgium’s University of Liege.  They’ve built a pair of robotic telescopes tailored for seeking out rocks and comets local to our Solar System.  Exoplanets, too.  Astronomers love tying catchy acronyms to their projects.  This group’s proudly Belgian so they called their robots TRAnsiting Planets and Planetesimals Small Telescopes, ergo TRAPPIST, to honor the country’s 14 monasteries.  And their beer.  Mainly the beer, I’ll bet.”

“So the planets are a Belgian discovery?”

“Well, the lead investigator, Michaël Gillon, is at Liege, and so are half-a-dozen of his collaborators.  Their initial funding came from the Belgian government.  But by the time the second paper came out, the one that claimed a full seven planets spanning a new flavor of Goldilocks Zone, they’d pulled in support and telescope time from over a dozen other countries — USA, India, UK, France, Morocco, Saudi Arabia… the list goes on.  So it’s Belgian mostly but not only.”

“I love international science.  Next question — I see the planets are listed as TRAPPIST-1b, TRAPPIST-1c, and so on up to TRAPPIST-1h.  What happened to TRAPPIST-1a?”

“Rules of nomenclature, Sy.  TRAPPIST-1a is the star itself.  Actually, the star already had a formal name, which I just happen to have written down in my seminar notes somewhere … here it is, 2MASS J23062928 – 0502285.  You can see why TRAPPIST-1 is more popular.”

“I’m not even going to ask how that other name unwinds.  So what was the seminar topic this week?”

7 planets
TRAPPIST-1’s planets,
drawn to scale against their star. The
green ones are in the Goldilocks Zone.

“The low probability for us ever noticing those planets blocking the star’s light.”

“I’d think seeing a star winking on and off like it’s sending Morse code would attract attention.”

“That’s not close to what it was doing.  It’s all about the scale.  You know those cartoons that show planets together with their host sun?”

(showing her my smartphone) “Like this one?”

“Yeah.  It’s a lie.”

“How is it lying?”

“It pretends they’re all right next to the star.   7 planets perspectiveThis image is a little better.”  (showing me her phone)  “This artist at least tried to build in some perspective.  Even in this tiny solar system, about 1/500 the radius of ours, the star’s distance to each planet is hundreds to a thousand times the size of the planet.  You just can’t show planets AND their orbits together in a linear diagram.  Now, think about how small these planets are compared to their sun.”

“Aaaa-hah!   When there’s an eclipse, only a small fraction of the light is blocked.”

“That’s part of it.  Each eclipse (we call them transits) dims the measured brightness by only a percent or so.  But it’s worse than that.”

eclipses“How so?”

“All those orbits lie in a single plane.  We can’t see the transits unless our position lines up with that plane.  If we’re as little as 1½° out of the plane, we miss them.  But it’s worse than that.”

“How so?”

“During a transit, each planet casts a conical shadow that defines a patch in TRAPPIST-1’s sky.  You can tile TRAPPIST-1’s sky with about 150,000  patches that size.  There’s one chance in 150,000 of being in the right patch to see that 1% dimming.  In our sky there are over 6×1015 patches the size of TRAPPIST-1h’s orbit.  The team had to inspect the just right patch to find it.”

“With odds like that, no wonder TRAPPIST uses robots.”

“Yep.”

~~ Rich Olcott