Philippa Kaur

If you’re an aspiring life-form, you might want to steer clear of planets around orange dwarf stars.

Some astronomers have called these orange suns “Goldilocks stars” (SN: 11/18/09). They are dimmer and age more slowly than yellow sunlike stars, thus offering an orbiting planet a more stable climate. But they are brighter and age faster than red dwarfs, which often spew large flares. However, new observations show that orange dwarfs emit lots of ultraviolet light long after birth, potentially endangering planetary atmospheres, researchers report in a paper submitted March 29 at arXiv.org.

Using data from the Hubble Space Telescope, astronomer Tyler Richey-Yowell and her colleagues examined 39 orange dwarfs. Most are moving together through the Milky Way in two separate groups, either 40 million or 650 million years old.

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To Richey-Yowell’s surprise, she and her team found that the ultraviolet flux didn’t drop off from the younger orange stars to the older ones — unlike the case for yellow and red stars. “I was like, `What the heck is going on?’” says Richey-Yowell, of Arizona State University in Tempe.

In a stroke of luck, another team of researchers supplied part of the answer. As yellow sunlike stars age, they spin more slowly, causing them to be less active and emit less UV radiation. But for orange dwarfs, this steady spin-down stalls when the stars are roughly a billion years old, astronomer Jason Lee Curtis at Columbia University and colleagues reported in 2019.

“[Orange] stars are just much more active for a longer time than we thought they were,” Richey-Yowell says. That means these possibly not-so-Goldilocks stars probably maintain high levels of UV light for more than a billion years.

And that puts any potential life-forms inhabiting orbiting planets on notice. Far-ultraviolet light — whose photons, or particles of light, have much more energy than the UV photons that give you vitamin D — tears molecules in a planet’s atmosphere apart. That leaves behind individual atoms and electrically charged atoms and groups of atoms known as ions. Then the star’s wind — its outflow of particles — can carry the ions away, stripping the planet of its air.

But not all hope is lost for aspiring life-forms that have an orange dwarf sun. Prolonged exposure to far-ultraviolet light can stress planets but doesn’t necessarily doom them to be barren, says Ed Guinan, an astronomer at Villanova University in Pennsylvania who was not involved in the new work. “As long as the planet has a strong magnetic field, you’re more or less OK,” he says.

Though far-ultraviolet light splits water and other molecules in a planet’s atmosphere, the star’s wind can’t remove the resulting ions if a magnetic field as strong as Earth’s protects them. “That’s why the Earth survived” as a life-bearing world, Guinan says. In contrast, Venus might never have had a magnetic field, and Mars lost its magnetic field early on and most of its air soon after.

“If the planet doesn’t have a magnetic field or has a weak one,” Guinan says, “the game is over.”

What’s needed, Richey-Yowell says, is a study of older orange dwarfs to see exactly when their UV output declines. That will be a challenge, though. The easiest way to find stars of known age is to study a cluster of stars, but most star clusters get ripped apart well before their billionth birthday (SN: 7/24/20). As a result, star clusters somewhat older than this age are rare, which means the nearest examples are distant and harder to observe. More

Fragmenting planets sweeping extremely close to their stars might be the cause of mysterious cosmic blasts of radio waves.

Milliseconds-long fast radio bursts, or FRBs, erupt from distant cosmic locales. Some of these bursts blast only once and others repeat. A new computer calculation suggests the repetitive kind could be due to a planet interacting with its magnetic host star, researchers report in the March 20 Astrophysical Journal.

FRBs are relative newcomers to astronomical research. Ever since the first was discovered in 2007, researchers have added hundreds to the tally. Scientists have theorized dozens of ways the two different types of FRBs can occur, and nearly all theories include compact, magnetic stellar remnants known as neutron stars. Some ideas include powerful radio flares from magnetars, the most magnetic neutron stars imaginable (SN: 6/4/20). Others suggest a fast-spinning neutron star, or even asteroids interacting with magnetars (SN: 2/23/22).

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“How fast radio bursts are produced is still up for debate,” says astronomer Yong-Feng Huang of Nanjing University in China.

Huang and his colleagues considered a new way to make the repeating flares: interactions between a neutron star and an orbiting planet (SN: 3/5/94). Such planets can get exceedingly close to these stars, so the team calculated what might happen to a planet in a highly elliptical orbit around a neutron star. When the planet swings very close to its star, the star’s gravity pulls more on the planet than when the planet is at its farthest orbital point, elongating and distorting it. This “tidal pull,” Huang says, will rip some small clumps off the planet. Each clump in the team’s calculation is just a few kilometers wide and maybe one-millionth the mass of the planet, he adds.

Then the fireworks start. Neutron stars spew a wind of radiation and particles, much like our own sun but more extreme. When one of these clumps passes through that stellar wind, the interaction “can produce really strong radio emissions,” Huang says. If that happens when the clump appears to pass in front of the star from Earth’s perspective, we might see it as a fast radio burst. Each burst in a repeating FRB signal could be caused by one of these clumps interacting with the neutron star’s wind during each close planet pass, he says. After that interaction, what remains of the clump drifts in orbit around the star, but away from Earth’s perspective, so we never see it again.

Comparing the calculated bursts to two known repeaters — the first ever discovered, which repeats roughly every 160 days, and a more recent discovery that repeats every 16 days, the team found the fragmenting planet scenario could explain how often the bursts happened and how bright they were (SN: 3/2/16).

The star’s strong gravitational “tidal” pull on the planet during each close pass might change the planet’s orbit over time, says astrophysicist Wenbin Lu of Princeton University, who was not involved in this study but who investigates possible FRB scenarios. “Every orbit, there is some energy loss from the system,” he says. “Due to tidal interactions between the planet and the star, the orbit very quickly shrinks.” So it’s possible that the orbit could shrink so fast that FRB signals wouldn’t last long enough for a chance detection, he says.

But the orbit change could also give astronomers a way to check this scenario as an FRB source. Observing repeating FRBs over several years to track any changes in the time between bursts could narrow down whether this hypothesis could explain the observations, Lu says. “That may be a good clue.” More