\n<\/p>\n
Astronomers have yet to find irrefutable proof for any natural satellites of exoplanets\u2014so-called exomoons\u2014but as circumstantial evidence accumulates and the list of candidates grows, the discovery of a true-blue exomoon seems to be looming on the horizon.<\/p>\n
The latest not-quite-smoking-gun claim concerns a potential exomoon that may be erupting to spew debris onto and around its host planet. Using NASA\u2019s James Webb Space Telescope (JWST), astronomers have identified a cloud of gas in the vicinity of the gas giant exoplanet WASP-39b that may come from an accompanying satellite. But even if that purported lunar companion proves illusory, this new method of tracking down mysterious sources of unexplained material around giant exoplanets could become a definitive pathway for future exomoon finds.<\/p>\n
Since its discovery in 2011, WASP-39b has been a frequent target for astronomers; the planet\u2019s large size, short-period orbit and shadowy transit (crossing the face of its star as seen from Earth) make it favorable for more in-depth studies. In 2023 researchers announced the detection of sulfur dioxide in the planet\u2019s atmosphere. Now a new preprint study, accepted for publication in the Monthly Notices of the Royal Astronomical Society, is challenging some details of that conclusion. The sulfur dioxide\u2019s source, this new study claims, is a hypervolcanic exomoon, similar in many ways to Jupiter\u2019s satellite Io. Caught in a gravitational tug-of-war between Jupiter\u2019s intense gravity and that of other nearby large moons, Io\u2019s innards are kneaded like dough by tidal forces, generating immense heat that powers enormous eruptions.<\/p>\n
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For this exoplanet and its putative moon, the process would be \u201cnearly identical with that of Io [and Jupiter] except that [WASP-39b] is very close to the star,\u201d says Apurva Oza of the California Institute of Technology, who led the new study. \u201cThe star is really cooking it, gravitationally and thermally as well.\u201d<\/p>\n
The most volcanically active body in the solar system, Io ejects material that is then swept into space by Jupiter\u2019s magnetosphere at a rate of roughly a ton per second. The result is a torus of gas, dust and other debris stretching around Jupiter, constantly replenished by Io\u2019s incessant eruptions.<\/p>\n
In 2006 researchers posited that similar clouds around exoplanets could reveal the presence of moons. Oza began working with one of those researchers a few years later, considering how sodium could become \u201ca beacon for exomoons and exorings.\u201d In 2019 he and his colleagues put together a list of potential targets to prioritize in hunting for such beacons. WASP-39b was one of the contenders.<\/p>\n
Although JWST was the first to spot sulfur dioxide around WASP-39b, other observatories such as NASA\u2019s Hubble Space Telescope and the European Southern Observatory\u2019s Very Large Telescope in northern Chile have detected sodium and potassium there as well. Oza and his colleagues pulled together all of the observations and tracked how they varied over more than a decade. Rather than being uniform and unambiguously associated with WASP-39b itself, the fluctuating signals of the compounds suggested episodic behavior to Oza and his colleagues, while the composition hinted at an external source.<\/p>\n
\u201cThe fact that these particular species are varying really points to something that\u2019s more of a solid body, like a moon would be,\u201d says Kurt Retherford, a planetary scientist at the Southwest Research Institute in San Antonio, Tex., who was not involved with Oza\u2019s study. At various professional meetings in recent years, Retherford has emerged as a persistent critic of Oza\u2019s preferred exomoon-hunting technique, showing up to Oza\u2019s talks to ask tough, skeptical questions.<\/p>\n
But that may have shifted since Oza presented the research about WASP-39b last month at the joint Europlanet Science Congress\/Division of Planetary Sciences (EPSC-DPS) conference in Helsinki, Finland.<\/p>\n
\u201cBefore I saw his talk, I would have leaned more toward the planet itself\u201d as the likely source of sulfur dioxide, Retherford says. Now, however, he\u2019s changed his mind. An external, nonplanetary source for strange readings from WASP-39b is more sensible, he says, \u201cmaybe with an exomoon being the best explanation for the data as it stands right now.\u201d<\/p>\n
Oza and his team are already applying the technique around other stars. They\u2019ve already found another transiting world, WASP-49Ab, they think is a strong candidate to host an exomoon. Like its sibling, WASP-49b is a \u201chot Jupiter\u201d\u2014a gas giant orbiting extremely close to its star. A cloud of sodium orbits the star in fits and bursts that suggest eruptions. That study was published last year in the Astrophysical Journal Letters.<\/p>\n
Recently, Oza\u2019s collaborator and co-author on the new study Athira Unni of the University of California, Santa Cruz, released measurements of the movement of the gas itself around WASP-49b, citing the rapid velocity around the system as a clue toward the origin being a volcanic satellite rather than stellar eruptions or other astrophysical sources. Those measurements appear to suggest a moon with an eight-hour orbit around its host planet, according to Oza. If observations of gas velocities around WASP-39b revealed similar patterns, Retherford speculates, \u201cthat would be the smoking gun as an exomoon.\u201d<\/p>\n
A composite view of Jupiter and its volcanic moon Io, assembled from images captured by NASA\u2019s New Horizons spacecraft during its Jupiter flyby in early 2007.<\/p>\n
But the signals aren\u2019t completely unambiguous. All of the targets on Oza\u2019s 2019 list are hot Jupiters. WASP-39b is about the size of Saturn and whips around its sunlike star once every four days; that broiling orbit heats the planet\u2019s dayside to 1,430 degrees Fahrenheit (776 degrees Celsius). WASP-49Ab orbits every 2.8 days and is even hotter. Most astronomers think these scorching giants formed farther away from their star and then migrated inward.<\/p>\n
That creates some problems when it comes to satellites.<\/p>\n
Planets form from the disk of gas and dust that surrounds a newborn star, and moons are thought to typically emerge from the dregs left over from a planet\u2019s birth. Giant planets coalesce farther out in the cooler regions of their stars\u2019 natal disks, where ice and gas are more abundant; hot Jupiters are thought to have been hurled inward soon after their formation. Their moons would have presumably formed alongside them. But the chaos that kicks hot Jupiters inward would most likely strip away their satellites, according to David Kipping, an exomoon-hunting astronomer at Columbia University.<\/p>\n
\u201cWe can think of lots of plausible ways for a moon to be lost,\u201d Kipping says. \u201cHolding onto it is hard.\u201d<\/p>\n
Fellow exomoon hunter Ren\u00e9 Heller of the Max Planck Institute for Solar System Research in G\u00f6ttingen, Germany, is less skeptical on that front. He argues that a less extreme inward migration probably happened around our own sun, too, and that Jupiter and Saturn managed to bring their largest moons along for the ride. \u201cOur solar system acts as an example for moving things inward and carrying their moons along with them,\u201d Heller says.<\/p>\n
Heller is more concerned about how close the moon would have to orbit around WASP-39b to avoid being stripped away by the star. According to Oza\u2019s analyses, the erupting orb would have to be within one planetary radius of WASP-39b\u2014practically skimming the planet\u2019s cloud tops. That\u2019s a fine line to walk without crashing into the giant or veering off into the sun.<\/p>\n
\u201cI think it’s very implausible,\u201d Heller says. \u201cStability is a tough criterion.\u201d<\/p>\n
Both Heller and Kipping also raise concerns about falsifiability and the need to test predictions. Planet-star interactions, they each independently argue, could be misinterpreted as lunar activity. Stellar activity could affect the gas cloud, causing episodic behavior that could mimic eruptions. Heller points to \u201cunknown unknowns,\u201d noting that the planet\u2019s proximity to its star could result in signal sources scientists have not yet discovered.<\/p>\n
\u201cI think it’s questionable whether we understand stars enough to confidently assert that any variability we see, spectroscopically especially, cannot be the result of some process happening on the surface of the star itself,\u201d Kipping says.<\/p>\n
One downside of Oza\u2019s model, Kipping notes, is that any fluctuations in the levels of sulfur dioxide, sodium and potassium observed on WASP-39b can be easily explained away as variability in eruptions.<\/p>\n
\u201cWhen your hypothesis can explain everything, it becomes very difficult to disprove it,\u201d he says.<\/p>\n
Heller expresses similar concerns about how to clearly determine whether material came from a moon rather than from a planet or some other source. But for Retherford, the origin seems fairly straightforward.<\/p>\n
\u201cIt’s more of a challenge for me to imagine the sodium, potassium and sulfur dioxide in the upper parts of the gas giant atmosphere,\u201d he says.<\/p>\n
The outermost atmospheric layer of the solar system\u2019s gas giants is typically filled with light elements such as hydrogen and helium. Heavier material sinks farther down and would be more challenging to strip off. This would presumably hold true for hot Jupiters as well, but certainty about such arcana is elusive for these far-distant worlds.<\/p>\n
Even with that in mind, Retherford says that the exomoon explanation provides \u201ca much simpler explanation\u201d for observations of the gas.<\/p>\n
What would it ultimately take to convince Heller, Kipping and other skeptics?<\/p>\n
\u201cI guess we would need a complementary method,\u201d Heller says, \u201can independent method to prove any predictions you have.\u201d<\/p>\n
One option is to further capitalize on the fact that WASP-39b transits its star as seen from Earth. A sufficiently hefty accompanying moon could, for instance, tug enough on the planet to subtly alter the timing of those transits, potentially allowing vigilant astronomers to infer its presence.<\/p>\n
Kipping points out, however, that over nearly a decade and a half, astronomers have already lavished attention on WASP-39b, and numerous transits have been captured. It is possible, he says, that the transit-timing signal for the new moon could yet be hidden within existing data. But Oza suspects that any such signal would be too small to be detected.<\/p>\n
\u201cThis technique is extremely sensitive to the mass of the moon, which could be smaller than our own moon or Io,\u201d he says.<\/p>\n
Meanwhile, Oza says, Unni\u2019s measurements for the velocity of the cloud around WASP-49Ab provide strong support for the existence of a moon there.<\/p>\n
\u201cHaving that extra dimension of Doppler shifts definitely strengthens the analysis,\u201d Kipping says.<\/p>\n
Oza hopes to obtain similar measurements of WASP-39b in the near future.<\/p>\n
\u201cIt\u2019s a really intriguing signal that demands an explanation, and an exomoon can explain it,\u201d Kipping says. \u201cI think we should pursue it.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"
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