Astronomers make first calculations of magnetic activity in 'hot Jupiter' exoplanets

July 22, 2019

Gas-giant planets orbiting close to other stars have powerful magnetic fields, many times stronger than our own Jupiter, according to a new study by a team of astrophysicists. It is the first time the strength of these fields has been calculated from observations.

The team, led by Wilson Cauley of the University of Colorado, also includes associate professor Evgenya Shkolnik of Arizona State University's School of Earth and Space Exploration. The other researchers are Joe Llama of Lowell Observatory and Antonino Lanza of the Astrophysical Observatory of Catania in Italy. Their report was published July 22 in Nature Astronomy. This illustration shows a hot Jupiter orbiting so close to a red dwarf star that the magnetic fields of both interact, triggering activity on the star. Astrophysicists have for the first time used observations of such activity to calculate field strengths in four hot Jupiter star-and-planet systems. Image credit: NASA, ESA and A. Schaller (for STScI) Download Full Image

"Our study is the first to use observed signals to derive exoplanet magnetic field strengths," said Shkolnik. "These signals appear to come from interactions between the magnetic fields of the star and the tightly orbiting planet."

Many worlds

More than 3,000 exoplanet systems containing over 4,000 planets have been discovered since 1988. Many of these star systems include what astronomers call "hot Jupiters." These are massive gaseous planets presumed to be like the sun's Jupiter but orbiting their stars at close distances, typically about five times the star's diameter, or roughly 20 times the moon's distance from Earth.

Such planets travel well inside their star's magnetic field, where interactions between the planetary field and the stellar one can be continual and strong.

Previous studies, the team says, have placed upper limits on exoplanet magnetic fields, for example from radio observations or derived purely from theory. 

"We combined measurements of increased stellar emission from the magnetic star-planet interactions together with physics theory to calculate the magnetic field strengths for four hot Jupiters," lead author Cauley said.

The magnetic field strengths the team found range from 20 to 120 gauss. For comparison, Jupiter’s magnetic field is 4.3 gauss and Earth's field strength is only half a gauss, although that is strong enough to orient compasses worldwide. 

Triggering activity

The astrophysicists used telescopes in Hawaii and France to acquire high-resolution observations of emission from ionized calcium (Ca II) in the parent stars of the four hot Jupiters. The emission comes from a star's hot, magnetically heated chromosphere, a thin layer of gas above the cooler stellar surface. The observations let the team calculate how much energy was being released in the stars' calcium emission. 

Said Shkolnik, "We used the power estimates to calculate magnetic field strengths for the planets using a theory for how the planets' magnetic fields interact with the stellar magnetic fields." 

Cauley explained, "Magnetic fields like to be in a state of low energy. If you twist or stretch the field like a rubber band, this increases the energy stored in the magnetic field." Hot Jupiters orbit very close to their parent stars and so the planet's magnetic field can twist and stretch the star's magnetic field.

"When this happens," Cauley said, "energy can be released as the two fields reconnect, and this heats the star's atmosphere, increasing the calcium emission."

Probing deep

Astrophysicists have suspected that hot Jupiters would, like our own Jupiter, have magnetic fields produced deep inside them. The new observations provide the first probe of the internal dynamics of these massive planets.

"This is the first estimate of the magnetic field strengths for these planets based on observations, so it's a huge jump in our knowledge," Shkolnik noted. "It's giving us a better understanding of what is happening inside these planets."

She adds that it should also help researchers who model the internal dynamos of hot Jupiters. "We knew nothing about their magnetic fields — or any other exoplanet magnetic fields — and now we have estimates for four actual systems."

Surprisingly powerful

The field strengths, the team says, are larger than one would expect considering only the rotation and age of the planet. The standard dynamo theory of planetary magnetic fields predicts field strengths for the sampled planets that are much smaller than what the team found.

Instead, the observations support the idea that planetary magnetic fields depend on the amount of heat moving through the planet's interior. Because they are absorbing a lot of extra energy from their host stars, hot Jupiters should have larger magnetic fields than planets of similar mass and rotation rate.

"We are pleased to see how well the magnitude of the field values corresponded to those predicted by the internal heat flux theory," Shkolnik said. "This may also help us work toward a clearer understanding of magnetic fields around temperate rocky planets."

Robert Burnham

Science writer, School of Earth and Space Exploration


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ASU researcher discovers rare rescue behavior in ants

July 22, 2019

Desert seed-harvesting ants will save nestmates from spiderwebs

It started out as a typical day at work for ant researcher Christina Kwapich.

Like other mornings, she set out before the sun came up to study Veromessor pergandei, a species of desert seed-foraging ants. On this day, she was counting the number of ants a colony needs to produce each year to survive.

When she did this, however, Kwapich, an Arizona State University School of Life Sciences postdoctoral research associate in Professor Bert Hölldobler’s lab, noticed something interesting about the ants’ behavior and realized she had never seen it before.

As the ants came out to forage at 4 a.m., so did the spiders. They would wander around through the desert until they discovered an ant trail, and then they would quickly build webs.

What happened next really caught her eye: If an ant was trapped, other ants would gather around the web and tear it down to rescue the victim.

“I just started seeing these little disruptions in the trail and there was all this drama,” Kwapich said. “It was like taking down a circus tent or something. They’re pulling on the legs of the web and the spider is going crazy and running away. Once they get the web pulled apart, the ant is in this giant tangle of threads and they carry it back to the nest.”

New context for an old behavior

While rescue behavior has been recorded in wild populations, only two species of primates, which will tear apart human snare traps, have been shown to actually destroy the traps as these ants do. In addition, this rescue behavior typically occurs in animals with small colonies, such as bottlenose dolphins. Kwapich's research, which is published in The American Naturalist, is the first to record this behavior in a large colony.

There are more than 16,000 ant species and rescue behavior has only been recorded in five, typically in smaller colonies, such as the Metabele ant, which produces only 13 new nestmates per day.

“The prevailing thought has been that if you have a very small society like chimpanzee group or some Atlantic bottlenose dolphins, then every individual in the group has a higher value,” Kwapich said. “So, if you have a group of 10 and you lose one, you lose 10% of your colony. That’s been true for many of the ants who show rescue behavior.”

But not this colony. These foraging ants produce more than 650 ants per day.

“Social rescue behavior is an astounding action in animal societies. Whenever such behavior is observed in social groups of vertebrates, for example in nonhuman primates or elephants, it causes a lot of excitement and may even be reported on national and international news channels,” said Hölldobler, a University Professor of Life Sciences at ASU. “Christina’s discovery of rescue behavior of foragers is exceptional because the number of foragers is tremendous. Foragers attempt and often succeed in freeing ensnarled nestmates and, in addition, remove the webs. This is an entirely new rescue behavior observed in social insects.”

Observing ants in a glass nest

To understand what was happening, Kwapich gathered up some of the ants and brought them back to the lab to observe them in an artificial glass nest. From there, she could see exactly how the ants dealt with the nests and what they did after rescuing a nestmate.

She observed some interesting behavior. First, she noticed that ants would only tear down a web if a nestmate was ensnared. These ants carry large seeds back to their nests. If one of these seeds gets stuck in a spider web, the ant will continue struggling to walk with that seed rather than give it up. However, it will never tear down the web.

Then, she observed that once an ant had been rescued, she is taken back to the colony, where the other ants gently remove the sticky web from her.

But what is the advantage of this meticulous effort they perform for one another — but not for large seeds?

Understanding this ant species

The Veromessor pergandei ant species has a unique foraging strategy: They leave the nest in a large swarm and march together to their foraging destination.

“It looks like a black tube of toothpaste coming out of the hole. If you see if from a distance, you might think it’s a snake,” Kwapich said. “You have like 30,000 ants and they’re packed very close together, traveling together on this one column. It’s a really dense resource for predators, so obviously it attracts them, but it also allows the ants to be more defensive because they’re in really close proximity to each other.”

This means that if an ant is in trouble and sends out a chemical distress signal, nearby ants are likely to detect it and be able to help, in contrast to ants that forage alone.

In addition, these ants are sensitive to heat. In the summer months, this leaves a narrow foraging window. Thus, changing the direction of the foraging trail is not an option. It’s easier to tear down a spider web that blocks the path than change the path entirely.

However, Kwapich also discovered the ants can’t see the webs. They only know it exists once receiving a chemical signal from the trapped ant. Since a trapped seed may cost one ant a whole day of work, tearing down the web is important, but they can only do this if another ant uses its defense signal to alert the colony to the web.

Saving sisters

Another interesting characteristic of Veromessor pergandei is that their foragers are often different sizes, with some being much larger than others. When a distress signal occurs, the larger ants are more likely to respond and take on the task of destroying the web. Thus, they may be specialized ant body guards.

And once the ants are rescued, they are returned to the nest and meticulously cleaned by their sisters.

Why all that effort for a colony that produces 650 ants per day?

“Even though it’s a big society, every ant matters because they have a very limited foraging window. Because of the heat, sometimes they only forage two hours a day, in contrast to other ant species that can forage all day long or all night long,” Kwapich said. “That’s such a limited time to forage and they need to bring in so many resources for the queen to produce 600 ants a day. Temperature, time and their group foraging behavior constrain how much they can bring in. So individual workers are worth more in this society.”

Top photo: Ants rescue a nestmate that is trapped in a spiderweb while foraging. Some vertebrate species and small ant colonies will rescue each other, but only a couple of primate species are known to destroy the traps that ensnare them. Photo by Christina Kwapich/ASU

Melinda Weaver

Communications specialist , School of Life Sciences