BAN #451: A new way to look for exomoons — search rogue planets

August 8 2022 Issue #451

August 08, 2022

Blog Jam

[ARE YOU KIDDING ME with this JWST image of the Cartwheel Galaxy? From Thursday’s article. Credit; ASA, ESA, CSA, STScI ]

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

As of the time I write this, there are no known exomoons — moons orbiting exoplanets, which are in turn planets orbiting other stars.

Finding them is tough. Some exoplanets are seen directly in images, but any moons would be too faint and close to the planet to spot. It’s possible, barely, to see them in transit; when a planet passes in front of its star during an orbit it blocks a tiny fraction of the star’s light, which is how the majority of exoplanets has been found. There are a couple of very interesting candidates — Kepler 1625b-i and Kepler 1708b-i, but both are marginal detections and nowhere near confirmed. There are other indirect ways to find them, but so far nothing has proven out.

[The transit method works if an object’s orbit is aligned such that we see it edge-on or nearly so; it blocks the light of its host once per orbit and we see a dip in light. Credit: Greg Loughlin (adapted by Phil Plait)]

So what if the problem is that we’re looking at planets orbiting stars?

What if, instead, we turned to rogue planets, ones that are free-floating in space with no host star?

The technical term for these is isolated planetary-mass objects, or IPMOs, but c’mon, rogue planets is a way awesomer term.

These are planetary objects that either form around a star and get ejected, or form on their own directly like stars, collapsing from a cloud of gas and dust. Ejecting a planet is not only not that hard but actually pretty common; it’s possible our solar system used to have five giant planets but one got tossed out as the other four migrated inward and outward due to mutual gravitational interactions when the solar system was young.

Quite a few rogue planets are known, including one, WISE 0855−0714, that’s the seventh closest interstellar object known to the Sun! In fact that’s why it was discovered; it’s cold and faint, and probably roughly the same size as Jupiter. If it were more than about a dozen light years away it would be too faint to see at all.

Rogue planets could have moons; simulations show that some moons can survive a planet being ejected into interstellar space, and they could form naturally along with the planet if it collapses directly from a gas cloud.

A study published in 2021 looked into this [link to paper], to see if it’s possible to detect a moon around such an object. They propose to use the transit method: If the moon’s orbital geometry is lined up such that we see it cross the plant, then the light we see from the planet will dim, and will do so on a regular cycle.

[Artwork of a large exomoon orbiting its gas giant host, based on observations of Kepler 1625-b-i. Credit: NASA, ESA, and L. Hustak (STScI)]

The idea is to look at younger rogue planets. For some hundreds of millions or even billions of years after they form they’ll be hot — planets form from accreting matter from space, essentially getting slammed over and over again by asteroids that can be huge. That generates a lot of heat, and once the planet’s all done forming it can be a long, long time for it to cool down.

These planets can glow, especially in infrared light. That makes them juicy targets to look for transiting moons. They note that, theoretically speaking, 10 – 15% of these gas giants could have moons. Also, 33 of the 57 known rogue planets are bright enough that a transit of a big (Titan or Ganymede-sized) moon would be detectable by JWST. These moons would tend to have short orbits, possibly days or less, so monitoring them could pay off pretty quickly.

There are two main problems. One is that these objects will be naturally variable in brightness. They can have weather, for example, and clouds that change their brightness over time, and distinguishing that from a transiting exomoon could be tough. The other is that it will still take a lot of observing time to watch them, since you have to monitor each one for a long time to see anything. That will be a very tough sell for JWST (very, very tough).

[The closest systems to the Sun, out to a little over six light years, including WISE 0855-0714. Credit: NASA/Penn State University]

Still, there’s some evidence this could work. For example, Spitzer Space Telescope observations of a binary rogue planet called 2MASS J1119–1137 AB showed a fading event that is consistent with an exomoon transiting one of the two planets. If real, it would be about 1.7 times the diameter of Earth, so pretty big. But this has never been confirmed.

The good news is that JWST will likely wind up observing some of these objects anyway because they’re of interest. For example, the aforementioned rogue planet WISE 0855 is scheduled to be observed by JWST in its first year! That could happen any time now. It doesn’t look to me that the observations will last long enough to see a moon, but it’s possible; a planet-sized moon could be seen in the data. Interesting.

We have very little evidence that any exomoons exist, but given how common moons are in the solar system, especially around the giant planets, it makes sense that exoplanets have them as well. The technology to spot them is maybe kinda sorta just barely enough to do so.

Maybe in the next few years we’ll spot one.

Et alia

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