#exomoons

Planet Growth While We Watch

Ever thought it would be amazing if we could go back and watch how our solar system was created, how did Jupiter get so big, why is there a load of asteroids between it and Mars, why are the rocky planets where they are etc etc.

While we cannot go back and watch the process, we can watch the process unfold, and with JWST, we are even starting to see the planets themselves form, accreting material as they orbit the baby star.

The orange arc is the accretion disk that is orbiting the planet, and the two oddly shaped objects are planets, with their own disk of material making them appear slightly out of shape.

But look closer that there's a triangular shaped emission between the two planets, and scientists at this point aren't entirely certain what it is, but suspect it is material that is then being pulled between the two planets, or a third unseen planet between them pulling material.

eas always, CW for possibly delusion inducing content. you know the drill.

this is an important post for me because galileans are the primary point of my research. they are the reason i started this blog and the reason i am interested in the otherworld in general. i’m going to format this post in a bit of a Q & A fashion, i think it will make it more orderly and easier to comprehend. more under the cut

Ask Ethan: Can We Find Exoplanets With Exomoons Like Ours?

“But, by far, the best possibility we have today is through direct measurement of a transiting exomoon. If the planet that's orbiting the star can make a viable transiting signal, then all it will take is the same serendipitous alignment to have its moon transit the star, and sufficiently good data to tease that signal out of the noise.

This is not a pipe dream, but something that has already occurred once. Based on data taken by NASA's Kepler mission, the stellar system Kepler-1625 is of particular interest, with a transiting light curve that not only displayed the definitive evidence of a massive planet orbiting it, but of a planet that wasn't transiting with the exact same frequency you'd expect orbit after orbit.”

If you want to find an exoplanet, the most successful methods are to look for the effect it has on the light from it’s parent star. But what about if you wanted to find an exomoon? There are some subtle effects at play, but if we think hard about what they might be, we can come up with a series of methods that could reveal an exomoon’s presence indirectly, and pinpoint exactly where and when we could look to try and detect one directly. Thought to be a great technique for the upcoming James Webb Space Telescope to take advantage of TESS data, we’ve actually succeeded once already, using the Hubble/Kepler combo!

An artist's illustration of a potentially habitable exomoon orbiting a giant planet in a distant solar system. NASA GSFC: Jay Friedlander and Britt Griswold

We've all heard about the search for life on other planets, but what about looking on other moons?

In a paper forthcoming in The Astrophysical Journal, researchers at the University of California, Riverside and the University of Southern Queensland have identified more than 100 giant planets that potentially host moons capable of supporting life. Their work will guide the design of future telescopes that can detect these potential moons and look for tell-tale signs of life, called biosignatures, in their atmospheres.

Since the 2009 launch of NASA's Kepler telescope, scientists have identified thousands of planets outside our solar system, which are called exoplanets. A primary goal of the Kepler mission is to identify planets that are in the habitable zones of their stars, meaning it's neither too hot nor too cold for liquid water - and potentially life - to exist.

Terrestrial (rocky) planets are prime targets in the quest to find life because some of them might be geologically and atmospherically similar to Earth. Another place to look is the many gas giants identified during the Kepler mission. While not a candidate for life themselves, Jupiter-like planets in the habitable zone may harbor rocky moons, called exomoons, that could sustain life.

"There are currently 175 known moons orbiting the eight planets in our solar system. While most of these moons orbit Saturn and Jupiter, which are outside the Sun's habitable zone, that may not be the case in other solar systems," said Stephen Kane, an associate professor of planetary astrophysics and a member of the UCR's Alternative Earths Astrobiology Center. "Including rocky exomoons in our search for life in space will greatly expand the places we can look."

The researchers identified 121 giant planets that have orbits within the habitable zones of their stars. At more than three times the radii of the Earth, these gaseous planets are less common than terrestrial planets, but each is expected to host several large moons.

Scientists have speculated that exomoons might provide a favorable environment for life, perhaps even better than Earth. That's because they receive energy not only from their star, but also from radiation reflected from their planet. Until now, no exomoons have been confirmed.

"Now that we have created a database of the known giant planets in the habitable zone of their star, observations of the best candidates for hosting potential exomoons will be made to help refine the expected exomoon properties. Our follow-up studies will help inform future telescope design so that we can detect these moons, study their properties, and look for signs of life," said Michelle Hill, an undergraduate student at the University of Southern Queensland who is working with Kane and will join UCR's graduate program in the fall.

The title of the paper is "Exploring Kepler Giant Planets in the Habitable Zone." In addition to Hill, who is the is lead author, and Kane, other contributors are: Eduardo Seperuelo Duarte from Instituto Federal do Rio de Janeiro in Brazil; Ravi K. Kopparapu from the NASA Goddard Flight Center in Maryland; Dawn M. Gelino from the NASA Exoplanet Science Institute at Caltech; and Robert A. Wittenmyer from University of Southern Queensland.

In simulations of an Earth-sized planet orbiting a Sun-like star, the atmosphere of a 0.4 Earth-radius moon was shown to affect the system’s spectrum, in ways that could be observationally identical to signs of disequilibrium on a single body. In other words, your fancy methane-and-oxygen “biosignature” might just be a moon jumping into the frame, and it’s probably impossible to tell the difference. Resolution is the main limiting factor - not just present resolution, but projected best-case-scenario resolutions. Direct imaging allows us higher resolutions than infrared spectroscopy, but clouds and hazes make interpreting those images a complicated task.

The authors suggest two methods for unambiguously crossing off an exomoon from your laundry list of possible false-positives:

First, you could look for absorption features that together exceed 100% where they shouldn’t. This would suggest that at least some of one of the species in question is actually on the planet itself.

Alternately, you could discover a single-species silver-bullet biosignature that’s abundant enough to be observable and spectroscopically unique enough to be identifiable. #soundsfakebutok

PAPER: Rein, H., et al. 2014, “Some inconvenient truths about biosignatures involving two chemical species on Earth-like exoplanets.” Proceedings of the National Academy of Sciences, 111, 19 (link)

Like what you see? Think we’re full of baloney? Let us know! And follow The TL;DR for more planetary science research summaries and commentary, without the jargon.

BONUS COMMENTARY BELOW, for those who don’t mind reading more...

A few study highlights, limitations, and opportunities for more work: 

The authors focused specifically on an Earth-like planet orbiting a Sun-like star. These are among the systems most likely to be habitable (regardless of detection). However, it seems the authors’ primary concern is the detectability and disentanglement of spectral signatures from multiple small bodies. That task is likely to be much easier with small stars, which are much more common and - so far - much more likely to host rocky planets. If we suspect that resolution and signal-to-noise ratios will be limiting factors, I’d be curious to see a comparable analysis around a fairly quiescent M-dwarf.

The authors gave the moon an oddly dense atmosphere, with a surface pressure of 1500 mbar (versus the Earth’s 1013 mbar). This fits nicely with their suggested method of detecting the moon’s interference - namely, looking for absorption features that together exceed 100% once you’ve disentangled the spectra. In a way, it also suggests that fainter signals might be harder to see for what they are (a lunar photobomb). But...

Just how big does a moon have to be to retain an atmosphere in the first place, assuming it formed alongside its planet (i.e. by impact, not capture)? Are habitable-zone Earth-like planets in stable orbits likely to have moons that size? In other words, is this particular moon - large, with a thick atmosphere - a strawman?

Open questions: If the moon didn’t form in situ, what are the odds of atmosphere-hosting moon capture by the largest possibly-habitable rocky planets? How long would those systems take to stabilize? Would the instability be detectable? And what would the stabilization process do to their atmospheres? Could instability or tidal heating, alongside heightened flux from inward migration, even create an atmosphere from previously locked-in material? What would be the loss rate of such an atmosphere on such a small body?

Most of all, I’d be curious to see mechanisms by which these degeneracies could be resolved - i.e. by obtaining the occultation and transit spectra of each body independently. I can imagine seeking and timing spectral variations of the planet-moon system as a whole and finding the end-members within, either as a way to resolve the satellite’s orbital period or as a follow-up to transit-based disentanglement like Teachey, et al. suggested this year. Unlike the more distant habitable planets orbiting sun-like stars discussed here, habitable-zone planets orbiting close to cool stars have short enough orbital periods to permit continual observation... with the reasonable expectation of observing a satellite occultation (if a large, atmosphere-retaining satellite is actually present). 

The Rise of the Super Moons

For young astronomers today, the idea that planets circle other stars seems as obvious and normal as knowing the Sun is the centre of our Solar System, but 40 years ago, while it seemed an obvious and likely fact, there really was no evidence for it.

Today, over 10,000 exoplanets have been detected, and while many of them await confirmation (as waiting for a signal to re-appear can take many years as the planet orbits its star), the evidence is unanimous, that planets are a common feature of star systems in our galaxy, at least out to a few thousand light years from us.

But do other planets have moons ? As astronomers in the 1980s were fairly sure there would be planets, Astronomers today can look at our Solar System, and at the systems they are finding elsewhere, and feel fairly confident that will be the case, but lacking the evidence to support it, at least until very recently.

It's hard enough trying to spot a planet orbiting a star, but trying to then detect a moon orbiting that planet is on a totally different order of magnitude.

Originally, the only way we could spot planets was by detecting the wobble in the star as they gravitationally interacted, these were all super hot Jupiter like planets. In the same way, trying to find the first moons is concentrating on the largest planets, further out from their host star (so they can be imaged and monitored), and for moons that are extraordinarily large.

The most massive moon in our Solar System of course orbits the most massive planet, Jupiter. Ganymede is 0.02 times the mass of Earth, compared to 0.01 of our moon, so twice as massive. But even this huge moon would not be detectable from such distances, it takes moons the size of Neptune to really get noticed, and we are starting to detect such instances, albeit disputed and probably a good few years from being able to be confirmed (or refuted).

Astronomers have spotted what they think are Neptune sized moons orbiting super Jupiter type planets orbiting Kepler 1625b and more recently have proposed a moon 1/3rd the mass of Neptune, orbiting Kepler 1708b.

The jury is still out, but with JWST nearing readiness and the constant improvement in techniques, it's almost certain that the first moons detected outside of our solar system, will be giant gas moons orbiting even larger gas giants.