Tag Archives: Astronomy on Tap Seattle

The search for Earth 2.0

Astronomers have to date discovered more than 3,700 exoplanets—planets in orbit around stars other than our Sun. With each discovery, someone wants to know if the newly discovered planet is like Earth.

Elizabeth Tasker

Elizabeth Tasker at Astronomy on Tap Seattle.

Elizabeth Tasker thinks that’s not a very good question. Tasker, associate professor at the Japan Aerospace Exploration Agency, Institute of Space and Aeronautical Science and author of The Planet Factory: Exoplanets and the Search for a Second Earth (Bloomsbury Sigma, 2017) gave a talk at the most recent edition of Astronomy on Tap Seattle. She said that some of the exoplanets confirmed so far have at least a little resemblance to Earth.

“Roughly one third of those are approximately Earth-sized, by which I mean their physical radius is less than twice ours,” Tasker said. News media often wish to leap from that to describing a planet as Earth-LIKE, but Tasker said we don’t have nearly enough information to make that sort of call. Our current methods of detecting an exoplanet can give us either its radius or its minimum mass, and a pretty good read of its distance from its host star.

“The problem is neither of those directly relates to what’s going on on the surface,” Tasker noted. Part of the challenge is what Tasker feels is the somewhat oversimplified notion of the “habitable zone” around a star, a band of distance in which liquid water—a key to life as we know it—could exist on a planet’s surface.

“Like all real-estate contracts, there is small print,” Tasker said. “Just because you’re inside the habitable zone doesn’t mean you’re an Earth-like planet. Indeed, of all the planets we’ve found in the habitable zone around their stars, there are five times as many planets that are very likely to be gas giants like Jupiter than have any kind of solid surface.”

Another misleading metric that has been used is something called the “Earth similarity index.” This method compared exoplanets to Earth on the basis of properties such as density, radius, escape velocity, and surface temperature.

“None of these four conditions actually measure surface conditions at all,” Tasker said, pointing out that the index didn’t take into account such features as plate tectonics, a planet’s seasons, it’s magnetic fields, greenhouse gases, or existence of water. We can’t observe any of those things about exoplanets yet. As an example of the flaws of the index, Venus came out at 0.9, pretty similar to Earth, which is at 1.0 on the zero-to-one scale. While Venus is about the size of Earth and is around the inner edge of the Sun’s habitable zone, its surface temperature could melt lead. Not very Earth-like, or habitable. It’s one of the reasons that the index is seldom used these days. So we don’t have much of a clue about conditions on any of the known exoplanets.

“Our next generation of telescopes is going to change that,” Tasker said. She noted that NASA’s James Webb Space Telescope is scheduled to launch next year, the ESA’s Ariel in 2026, and the UK’s Twinkle in the next year or so.

“All of these are aiming at looking at atmospheres, and these may be able to tell us what is going on on the surface, and may even give us the first sniff of life on another planet,” Tasker said. “Maybe then we’ll be able to talk seriously about Earth 2.0.”

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Astronomy on Tap and a blue moon this week

It’s a light calendar of astronomy events for Easter week, but you can celebrate Astronomy on Tap Seattle’s third birthday and enjoy our second blue moon of the year!

Happy three to AOT

AOT March 28It’s hard to believe it’s been three years since a group of graduate students in astronomy started up the Astronomy on Tap Seattle lecture series, but this week’s edition will mark the 36th consecutive month that they’ve offered interesting talks, astronomy trivia, fun prizes, and great beer. Head to Peddler Brewing Company in Ballard at 7 p.m. Wednesday, March 28 for updates on the astronomy AOT has covered in the last year, and a look at the exciting new science that has come out recently—neutron star mergers, new planets, and more!

It’s free, but buy some beer. Bring your own chair to create premium, front-row seating.

Blue moon

It turns out “once in a blue moon” isn’t all that rare! Saturday’s full moon will already be the second one this year, at least by the definition that a blue moon is the second full moon in a calendar month. We had a blue moon in January, too; see the video below of Seattle Astronomy’s Greg Scheiderer talking on KING-TV’s New Day Northwest about the super blue blood moon.

The next blue moon after this week will be on Halloween in 2020.

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Exploring alien moons

The search for extraterrestrial life keeps getting smaller in scale. It’s difficult to discover planets around other stars, but now scientists are looking for exomoons and alien bacteria. Two University of Washington graduate students shared their work at an Astronomy on Tap Seattle gathering at Peddler Brewing Company in Ballard last week.

To date more than 3,500 exoplanets have been discovered in orbit around stars other than our Sun, but we haven’t seen an exomoon orbiting any of those planets. Tyler Gordon, a second year grad student in astronomy and astrobiology at UW, thinks it’s only a matter of time before we do.

“We have every reason to expect that there are a lot of moons out there in the universe, probably many more than there are exoplanets,” Gordon said. Thinking about our own solar system, he pointed out that there are 19 moons that are big enough to be rounded by their own gravity, which is more than twice as many such moons as there are planets.

Despite the fact that no exomoons have yet been found, Gordon said there are three good reasons to look for them:

  • They might be habitable
  • The presence or absence of a moon can give a clue about how a planet formed
  • A moon can be a factor in a planet’s habitability

There are two reasons for that third point. Moons can raise tides, and some scientists think that tides battering the shore on early Earth delivered nutrients and created places for life to develop. In addition, moons can influence a planet’s orbital characteristics, especially obliquity, and help stabilize oscillations of its rotational axis.

“An exomoon can keep a planet from tumbling back and forth onto its side and can insulate it from having really extreme changes in seasons, which is something that we think could be very bad for life,” Gordon said.

Where are they?

It’s been hard enough to identify exoplanets, and there’s an obvious challenge in hunting for exomoons.

“Exomoons are probably really small, and small is a problem because small things are really hard to see,” Gordon noted.

Just how small are moons? Gordon explained that observation and modeling have found that the mass of a planet’s satellites generally scales with the mass of the planet itself. For any given planet, “We expect that the total mass of its satellites adds up to between one ten-thousandth and two ten-thousandths of the mass of their host,” Gordon said.

Thus to find an exomoon as big as Earth—someting we could actually see—its host planet would have to be about 30 times the mass of Jupiter. Something that big probably wouldn’t be a planet; it would more likely be a brown dwarf.

Finding an exomoon

Most of the exoplanets discovered to date have been spotted because they cause a dip in the light we see coming from their star when they transit in front of it. An exomoon would do the same thing, but Gordon said there are a couple of challenges. Since the exomoon would be far smaller than the planet, so would the dip it would cause. Plus, sometimes the exomoon will be ahead of, behind, or blocked by the planet, making patterns more difficult to tease out of the data.

Gordon noted that a team from Columbia University recently tried to do that by looking at a ton of Kepler data and searching for scattering called the “orbital sampling effect.” In all of the data they found exactly one potential signal for an exomoon; it’s in orbit around the planet Kepler-1625b. This is a big planet, about ten times the mass of Jupiter, and the moon—Kepler-1625bi—is about the size of Neptune, which is way bigger than would be expected according to the scaling rule. Gordon said that raised a lot of questions.

“Is Kepler-1625b even a planet if it’s that’s big? Is the moon actually even there at all?” he asked. “And if it is there, how can such a large moon form?”

The Columbia team used the Hubble Space Telescope to look at the system back in October, but analysis of the data and any answers to those questions have yet to be published.

Gordon said that the James Webb Space Telescope will be a big help to exomoon hunters. While Kepler looked in the visual, JWST is equipped for other wavelengths.

“By using JWST’s ability to see in different parts of the electromagnetic spectrum we may be able to disentangle the transit of an exomoon from stellar variability that could obscure that transit,” Gordon said.

Space bacteria

Max Showalter is looking for stuff way smaller than exomoons. He’d like to spot interplanetary bacteria.

Showalter, a Ph.D. student in oceanography at the UW, gave a talk titled, “Looking for Life When the Trail Goes Cold.” He noted that the hunt for biosignatures is at the heart of the search for life. Biosignatures can be chemical, say oxygen in an atmosphere. They can be structures, such as fossils. They could be biological molecues like amino acids or nucleotides.

“They tell us that either life has been there in the past, or life is there now, or life could be there in the future,” Showalter said. “We want lots of biosignatures all telling us the same thing in order for us to decide that there’s life.”

Showalter would add another biosignature to the list: movement. After all, there’s no better sign of life than if something comes up and waves at you. Still, when you’re looking for microbial life, the tough questions are whether bacteria swim in space, and how we’ll see them if they do.

“It’s hard enough to see microbes on Earth, let alone millions of miles away,” Showalter noted. His research specialty is studying things that live in sea ice. When salt water freezes, the salt can either fall out or get stuck inside the ice. If it’s inside, the salt gets concentrated and melts pockets of ice, creating what are called brine pores.

“These brine pores are great because they make a really good habitat for a lot of things to live inside the ice,” Showalter said.

Ice-beings on Earth

Showalter has looked at bacteria from Arctic sea-ice on site with a microscope called SHAMU, which stands for “Submersible Holographic Astrobiology Microscope with Ultra-resolution.” SHAMU works on principles of holography. A laser in a box is split into two beams. One beam goes through the brine sample, the other goes straight to a camera. The waves of light interfere with each other, and computer analysis can create a hologram: “A 3-D image of a tube of liquid where you can see bacteria swimming,” Showalter said. (Read a longer article about SHAMU from a talk Showalter gave at Town Hall Seattle in 2016.)

The application for SHAMU in the search for life is at places like Saturn’s moon Enceladus and Jupiter’s moon Europa, both of which have salt water oceans under thick crusts of ice. Some of this salt water shoots out of geysers on the moons and into space.

“Which presents a really incredible opportunity for us as astrobiologists—or astronomers if you’re one of those people—to be able to sample that ocean without having to drill through eight kilometers of ice!” Showalter said.

An upcoming NASA mission called Europa Clipper will take a shot at that. The spacecraft is presently scheduled for launch some time between 2022 and 2025. SHAMU isn’t going, and none of the instruments selected for the mission will be looking for bacterial motility, but Showalter holds out hope that motility will prove to be a useful biosignature in the future.

He said he doubts that Europa Clipper will find life, but expects it will come across some tantalizing clues.

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Calendar: SAS banquet and Astronomy on Tap Seattle this week

The annual Seattle Astronomical Society banquet and Astronomy on Tap Seattle are the highlight events for the coming week. The Museum of Flight kicks off Astronaut Remembrance Week, and regional planetarium shows cap the calendar.

SAS Banquet

Robert Reeves

Robert Reeves

The Seattle Astronomical Society banquet always draws an excellent guest speaker, and this year is no exception: renowned photographer Robert Reeves will keynote the annual banquet, and talk in particular about observing and imaging the Moon. The banquet gets under way at 4 p.m. Sunday, January 28 at the Swedish Club on Dexter Avenue North in Seattle. Reservations are $65 for the general public, $55 for SAS members. Don’t wait; there were only 18 spots left as of this writing. Reservations are available online.

Reeves will do a special master class on lunar photography for the SAS Astrophotography Special Interest Group. The class is open to the public and will be held at 5:30 p.m. Saturday, January 27 in the Red Barn Classroom at the Museum of Flight.

Astronomy on Tap Seattle

AOT Seattle January 2018The topic will be exploring alien moons when Astronomy on Tap Seattle holds its first event of the new year at 7 p.m. Wednesday, January 24 in the beer garden at Peddler Brewing Company in Ballard. Second-year UW graduate student in astronomy and astrobiology Tyler Gordon will speak about his research on the search for exoplanetary satellites using current and future telescopes. UW Ph.D. student in oceanography Max Showalter will discuss looking for life when the trail goes cold, an update on his work using movement as a sign of life in icy places.

Showalter did a talk at Town Hall Seattle almost two years ago. Check our recap of that talk and learn how SHAMU is helping hunt for ET.

Planetarium shows

The Washington State University Planetarium in Pullman has a new show this week titled, “Millions of Miles to Mars.” The show explores the whats, hows, and whens of Mars visits. Showtimes are 7 p.m. Friday, Jan 26, and 5 p.m. Sunday, Jan 28. Tickets at the door are $5 cash or check; they don’t accept credit cards. Kids under six get in free.

The Willard Smith Planetarium at the Pacific Science Center has a variety of shows for all ages every day. Check their website for the complete calendar.

Astronaut remembrance

America’s three great spacefaring tragedies all occurred at this time of year. To honor the sacrifices of the fallen astronauts, the Museum of Flight holds an annual astronaut remembrance week. The event runs from Friday, January 26 through Sunday, February 4 and features displays and exhibits about the fallen astronauts and their accomplishments. Solar System Ambassador Ron Hobbs will give a presentation about the tragic missions, and about the risks and successes of space travel, at 2 p.m. Saturday, January 27. It’s free with museum admission.

Future file

A total eclipse of the Moon will be visible in the early morning hours of Wednesday, January 31. The event begins just after 3 a.m. PDT, the partial eclipse starts around 3:45, and it will be total from just before 5 a.m. until a little after 6:00. All you really need to do is go outside and look up, but if you want to watch with others, the Seattle Astronomical Society plans a group viewing event at Solstice Park in West Seattle.

You can always scout out future events on our calendar.

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Merging neutron stars and cool galaxies at Astronomy on Tap Seattle

One of the cool things about the Astronomy on Tap Seattle series of talks in pubs is access to scientists who are working on headline news. It happened at their October gathering at Peddler Brewing Company in Ballard. Jennifer Sobeck, a stellar astrophysicist in the Department of Astronomy at the University of Washington, was all set to give a talk titled, “A Hitchhiker’s Guide to the Galaxy: Bumming Around the Milky Way.” But a few days before the talk the news hit that LIGO and others had detected gravitational waves generated by merging neutron stars. Neutron stars are Sobeck’s thing, so the script went out the window and we learned about what happened.

Sobeck noted that neutron stars are what’s left behind when high-mass stars—around four to eight times the mass of the Sun—blow up in a supernova. Neutron stars are incredibly dense; the mass of the Sun packed into something 12 miles across. They have a crust, though light still gets through.

“Inside is just basically a soup,” Sobeck said. “It’s a hot mess.”

Everything inside is so compressed that scientists call it “degenerate.”

“There are no more atoms, there are no more molecules, those are all blown apart,” Sobeck explained. “It’s just like a soup of neutrons; there are just tons of neutrons, and the really cool thing is down in the center, they think the pressures are so high that you actually might get quarks.”

Neutron star merger animation by ESO/L. Calçada. Music: Johan B. Monell

August discovery

Scientists knew they had detected a neutron star merger rather than the sort of black-hole mergers previously spotted by LIGO because the signals are different. The interesting thing about the detection of two neutron stars merging is that we could see it visually because the event created a kilonova, like a supernova, but smaller.

“It’s a little bit less on the explosion scale,” Sobeck said. “Kilonova means that you’re able to have electromagnetic radiation across the spectrum that a whole bunch of facilites were able to monitor.”

So when LIGO and VIRGO detected the gravitational wave, with the help of the Fermi gamma ray space telescope and the ESA’s Integral gamma-ray observatory they they were able to narrow down the location of the event and tell others to look there. When the optical observations came in, the kilanova was spotted in the galaxy NGC 4993.

“This has never been done before,” Sobeck noted. The detection occurred in mid-August of this year, and by the end of the month the visual was gone.

“This kilonova explostion lasted only for a period of only 15 days,” Sobeck said.

Observations were made not just in the visual, but across the spectrum from gamma rays to radio, and more than a dozen observatories were involved in the analysis.

“You’re getting a different piece of information from all of these parts of the spectrum,” Sobeck noted. “They all helped fill in that puzzle.”

The story in the media

Where stuff comes from

Periodic table showing origin of elements in the Solar System, by CMGLee on Wikimedia Commons based on data by Jennifer Johnson, Ohio State University.

Sobeck said the press went a little overboard with headlines such as collision “creates gold” (CNN)  and “Universe-shaking announcement” (New York Times), yet it’s true that the kilonova made some gold. Sobeck showed the periodic table of elements at left (click to see it bigger), modified to show where all of the elements came from. She noted that hydrogen, helium, and a bit of lithium came from the Big Bang, the rest were made in stars. But stars can only fuse elements as heavy as iron. To get the really heavy stuff called lanthanides you need a kilanova. The emitted light tells you what’s there. If you see blue light after a kilonova, that means there’s a high concentration of silver, cadmium, and tin. If the light is more red, then platinum, gold, mercury, or lead is present.

“This particular event went from blue very, very, quickly to red, and it stayed red most of the time,” Sobeck said. “Hence, we’ve got a bunch of gold on our hands.”

“We found out that neutron-star mergers do make elements,” she said. “We were right, so huzzah!”

All kinds of galaxies

Grace Telford, a graduate student studying astronomy and data science at the UW, stuck with her original topic of “A Whirlwind Tour of Galaxies: the Tiny, the Gigantic, and Everything in Between” for the October Astronomy on Tap. She noted that there are several ways to classify galaxies:

  • Stellar mass or brightness
  • Shape
  • Star formation rate
  • Nuclear activity

Stellar mass or brightness

This is pretty straightforward.

“Basically the more stars a galaxy has, the brighter it is,” Telford noted. There’s quite a range of sizes. The Milky Way is a pretty common-sized galaxy, and it’s hard to make them bigger. The largest are around 10 times the size of the Milky Way.” Smaller galaxies are plentiful.

“A dwarf galaxy is something that is at least a hundred times less massive than our Milky Way,” Telford said, and they can go a lot smaller.

Way out at the small end of the chart are ultra faint dwarf galaxies, which can’t really be seen because they’re too faint. They can’t be detected at long distances.

A recently discovered type is called an ultra diffuse galaxy. This may be the same size as the Milky Way but have 100 times fewer stars, all held together by dark matter.

“This is an open area of research,” Telford said. “It’s hard to explain how to form these wierdo galaxies that are not very massive at all, but huge.”

Shape

The three main shapes of galaxies are elliptical, spiral, and irregular. Spirals may come with a large central bulge or a bar. Irregular galaxies tend to be small.

Star formation rate

It’s in star formation rate that galaxies really differentiate themselves, Telford said. Galaxies that emit a lot of blue light have lots of young stars and new star formation. Galaxies that look red are “quenched.” Their stars are older, and there’s little new star formation.

In between red and blue is the “green valley” of galaxies. They don’t actually emit green light, but they’re in transition from blue to red.

An interesting type is the “starburst” galaxy. These are galaxies that somehow stumble into a source of gas that wasn’t available to them before.

“They have the ability to form stars at a very high rate relative to the normal amount of star formation for a galaxy of its size,” Telford explained. “As a result, you have a lot of these massive young stars that are dying and exploding as supernovae and injecting a lot of energy into the gas.”

These objects are short-lived, they exhaust their gas in a hurry, at least in astronomical terms—in between 100 million years and a billion years.

Nuclear activity

Most galaxies have supermassive black holes, which can create jets of energy.

“Sometimes these black holes eat a lot of gas really quickly and then they blow out a whole bunch of energy,” Telford explained. These jets are nuclear activity. Galaxies with active galactic nuclei are most typically found in the green valley, though they’re in other types as well.

Telford gave a plug for Galaxy Zoo, where you can go looking for these differing types of galaxies and actually participate in citizen science.

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Seeing the invisible and finding aliens using polarimetry

The topic line for last week’s gathering of Astronomy on Tap Seattle was What the Hell is Polarimetry?, and it seemed that a significant portion of the audience at Peddler Brewing Company in Ballard shared the question.

UW postdocs Jamie Lomax and Kim Bott explained that when light starts from its source the oscillation of its wave—its “wiggle”—goes in all directions until an interaction with something makes it polarized.

“That just means that it’s wiggling in one direction,” Lomax noted. “There’s a preferred plane for that wiggle to happen in, and in polarimetry what we’re doing is measuring that preferred plane and we’re looking for light that has been polarized.”

“It can help you figure out the shape of things without having to resolve the object,” Bott added.

Polarimetry and massive stars

Lomax studies massive stars and has found use for polarimetry in her work. She gave a talk titled, “Seeing Invisible Circumstellar Structures.”

Jamie Lomax

Jamie Lomax

“The holy grail for us in massive star research is to be able to take a massive star at the beginning of its lifetime, figure out how massive it is,” Lomax said, “and map out what its life is going to look like and figure out what supernova it’s going to end its life as.”

“It turns out that is really hard, and it’s complicated by the fact that most massive stars are probably in binary systems,” she added. Since about two-thirds of massive stars are part of a binary system, one might expect that two-thirds of core-collapse supernovae would be from such systems.

“There’s a problem, and that is we’ve only seen maybe two or three core-collapse supernovae where we have evidence that suggests that it’s come from a binary star,” Lomax said.

Part of the problem, she said, is that we don’t yet know enough about the evolution of binary star systems.

“We can try to hammer out the details of how that mass is transferring between the two stars and when the system is losing material to try to figure out how that effects its future evolution,” Lomax said. “Once we start answering questions like that we can start to tease out why we aren’t seeing all of these binary supernovae we think we should be seeing.”

Lomax talked about the star Beta Lyrae, a binary system. The primary star in the system is losing mass that gets gobbled up by the secondary. This transfer of mass also forms a thick accretion disk of gas around the secondary—so thick light from the actual star can’t get through. There’s also evidence that there are jets shooting out of the system, but we don’t know where they are.

“These are all features that we can’t see very well,” Lomax said. “We can’t see the mass transfer stream between the two stars, we can’t see the jets.”

Here’s where polarimetry comes in. If a star is surrounded by a cloud of gas or dust that is circularly symmetrical, when the starlight interacts with that material the light becomes polarized, and the wiggles line up tangentially with the edge of the disk. If the cloud is elongated in some way, the wiggles form in a “preferred” direction.

“That preferred wiggle direction is 90 degrees from the direction of the elongation of the disk, so you can back out geometric information pretty quickly,” Lomax said. “Just by looking at how the light is wiggling I can tell you how the disc is oriented on the sky.”

Lomax figures that if you don’t do polarimetry you’re throwing out free information.

“You can see invisible things—to you—and that gives you extra information about what’s going on in different systems.”

Exoplanets and aliens

Bott’s talk was titled “The Polarizing Topics of Aliens and Habitable Planets.” She studies exoplanets and said polarimetry comes in handy.

“Stars don’t produce polarized light, which is really great if you’re trying to look at something dim like a planet,” she noted. The polarimeter will simply block out the starlight. There are then a number of things that might be spotted on the planet:

  • Glint from an ocean
  • Rayleigh scattering
  • Clouds and hazes
  • Rainbows
  • Biosignatures of gases in an atmosphere
  • Chiromolecules
Kim Bott

Kim Bott

These can help astronomers characterize a planet, judge its potential habitability, and even determine if life might already be flourishing there.

Bott said that polarimeters that are sensitive enough to study planets are a recent advance, and they’re studying big, bright planets to get the hang of it. Looking for rainbows can be revealing about liquids in the atmosphere of a planet.

“The light will bend in the droplets at a slightly different angle depending what the droplet is made out of,” Bott said, so they can tell whether its water, methane, or sulfuric acid.

“We’re trying to create these really robust models that will take into consideration polarized light from Rayleigh scattering in the atmosphere as well as from rainbows,” Bott said, “and if you have a planet where you can see the surface you’d be able to see the signature from glint as well.”

Since different substances bend light at different angles, we can also learn a lot by watching closely as planets move through their phases as they orbit their host stars.

“On Earth we have light going from air and bouncing off of H2O water,” Bott said. “That’s going to produce a maximum in polarized light at a different angle than on, say, Titan, where you have light going from a methane atmosphere and then bouncing off of a hydrocarbon ocean.”

“We can actually, in theory, tell what the ocean and atmosphere are made out of by looking at where, exactly, in the orbit we see this glint,” Bott explained.

As for aliens, life requires more complex molecules, chiromolecules, that are “wound” in a certain direction, like our own DNA. Such molecules would produce circularly polarized light, which if detected could be a sign that such molecules exist on the planet.

Astronomy on Tap Seattle is organized by graduate students in astronomy at the University of Washington. It’s next gathering is scheduled for October 30 at Peddler Brewing Company in Ballard.

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Learning about exoplanets with AoT Seattle

Often when an exoplanet is discovered the first question asked by the mainstream media is whether the new planet is “Earth-like.” In truth we know little about these far-away planets other than their mass or size, and whether they orbit within the habitable zone of their host star. Scientists are using PCA and SAMURAI to learn more about exoplanets, and LUVOIR may ultimately help us get a much better look at these distant worlds.

Tovar

Lupita Tovar spoke about mapping exoplanets at Astronomy on Tap Seattle August 23, 2017. (Photo: Greg Scheiderer)

Lupita Tovar is a first-year Ph.D. student in astronomy and astrobiology at the University of Washington, where she works at the Virtual Planetary Laboratory. She gave a talk titled “Mapping New Worlds” at the most recent Astronomy on Tap Seattle event at Peddler Brewing Company in Ballard. Tovar is helping develop the parameters for LUVOIR, which stands for Large UltraViolet/Optical/InfraRed Surveyor. It is one of four projects being considered by NASA as part of the 2020 decadal survey, which will help pick the agency’s next big project.

Big is the operative word for LUVOIR. Astronomers love aperture for their telescopes, and LUVOIR would dwarf any space telescope to date. The Hubble Space Telescope has a 2.4-meter mirror, and the James Webb Space Telescope, scheduled for launch next year, will be 6.5 meters. LUVOIR would nearly double that; Tovar said it is proposed right now to have a 12-meter mirror. It would also be equipped with a coronagraph which would block the light of a host star. Much as Venus and Mars were visible in the daytime during last month’s total solar eclipse, blocking starlight would allow us to see much dimmer objects nearby.

LUVOIR

Sketch of LUVOIR by NASA/Goddard

“The coronagraph will allow us to see those close-in planets, like Venus, and allow us to study those planets,” Tovar said. LUVOIR would be a powerful instrument. It could see Venus, Earth, and Jupiter from a distance of ten parsecs, or about 33 light years.

Fortunately, astronomers don’t have to wait for LUVOIR to make progress on mapping exoplanets. Tovar said that today they’re using PCA—Principal Component Analysis—to get a better idea about an exoplanet’s surface.

“We use PCA to extract how many components are there,” Tovar explained. “Is it just one, solid icy body? Are there two different types of surfaces sitting on that planet? Are there three? Are there more? PCA allows us to extract that information.”

Call in the SAMURAI

Once they know how many surface types there are, astronomers can then use SAMURAI—Surface Albedo Mapping Using RotAtional Inversion—to figure out just what those surfaces are. Tovar said it’s like looking at a beach ball as it is batted around. As the ball spins, different colors face the observer. SAMURAI uses algorithms to determine the composition of each surface type. For example, land reflects more light than ocean does, but an ocean’s reflection will spike when it’s near the edge of the exoplanet, from our view, because of the glint of light from the host star.

LUVOIR is just a glint in the eyes of astronomers now, but it along with PCA and SAMURAI could give us a much better idea about the makeup of exoplanets.

“Combined together, all of these three components will help you create a map,” Tovar said.

Is Tatooine out there?

Star Wars fans often speculate about the existence of planets like Luke Skywalker’s home world Tatooine, which has two suns. So far we know of a dozen exoplanets in orbit around binary star systems. Diana Windemuth, also a Ph.D. student in astronomy and astrobiology at UW, studies these sorts of systems and gave a talk titled, “By the Light of Two Suns” at Astronomy on Tap Seattle.

Windemuth

Diana Windemuth discussed exoplanets orbiting binary star systems at AoT Seattle. (Photo: Greg Scheiderer)

“Our Sun is a bit of a weirdo in that it does not have a companion,” Windemuth said, explaining that about half of stars like the Sun have one. The more massive a primary star is, the more likely it is to have a companion, she said. Further, there are two types of stable orbits a planet in a binary star system can have. In an S-type orbit the planet will go around just one of the stars; it will be either a circumprimary or circumsecondary orbit. In the P-type, the exoplanet orbits both stars.

“A circumbinary planet goes around in a wider orbit around an inner, closer-in binary,” Windemuth explained. She said it is harder to find these sorts of systems using Kepler’s transit method because throwing in a third body complicates things. Kepler measures the overall light from a system, and the amount of light we see changes not only when the planet transits, but when the stars eclipse each other.

“These are called eclipsing binaries because they go around one another,” Windemuth said. Exoplanets are confirmed when dips in the light during transits happen at regular intervals. Usually a computer picks that out of the data, but it doesn’t work so well on binary systems.

It’s a trick!

“It turns out its difficult to train a computer to do that because of what we call the geometric effect,” Windemuth said. Because the stars move with respect to each other in binary systems, the period of transits can appear to vary because of differing distances the light travels to reach us. Gravitational interactions in the system can also create wobble and change the perceived period of transits.

“Even though the period of your planet might be the same, the transits will occur at different times,” Windemuth noted.

It’s probably because of these challenges that we’ve only discovered a handful of circumbinary planets so far, Windemuth said, and none of them are candidates to be the real-life Tatooine.

“No terrestrial circumbinary planets have been found yet,” she said. That could be because they’re too hard to find, or maybe planets with short periods are destroyed when they orbit too close to the binary stars.

“It’s probably because our detection algorithms are not good enough yet,” Windemuth concluded.

Astronomy on Tap Seattle is organized by graduate students in astronomy at the University of Washington. The next gathering is scheduled for September 27 at Peddler Brewing Company in Ballard, and the topic will be polarimetry. We don’t know what that is, either, but are looking forward to finding out!

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