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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