The red dwarf star Proxima Centauri is our closest neighbor in space, so its two planets, which are only around four light-years away from us, could be the first targets of future interstellar space missions. But as has now been confirmed, her star is anything but calm: Astronomers observed an extreme radiation eruption from Proxima Centauri in May 2019. It only lasted a few seconds, but it released enormous amounts of energy in the form of radiation. In the radio and millimeter range, this was even the strongest eruption of this star ever detected. However, this is less good news for the life-friendliness of the planets around Proxima Centauri.
The star Proxima Centauri is only about four light years away from us. Accordingly, astronomers have searched intensively for planets there – with success. It is now clear that our neighboring star has two planets. The outer planet, Proxima Centauri c, is a super-earth about seven times the mass of the earth, which orbits the star outside the habitable zone. This is different with Proxima Centauri b, the inner of the two planets. It is about the size of the earth, orbits in the habitable zone of its star, and may also have liquid water. In this respect, this earth-like planet could possibly even be life-friendly. Because Proxima Centauri b is much closer to its star than Earth is to the sun, but it is a red dwarf – a star that is significantly smaller and cooler than our sun. Three quarters of all stars in the solar neighborhood belong to this spectral type.
Stellar outbreak on all channels
The problem, however, is that many red dwarfs are very active and often generate powerful bursts of radiation. This releases bursts of high-energy radiation that could potentially be fatal to life on nearby planets. The atmosphere of such a planet could also be thinned or torn away by the radiation and particle storms in the course of time. Astronomers have therefore been discussing for a long time whether life on exoplanets around red dwarfs could even form or last. This also applies to Proxima Centauri. It is known from our neighboring star that there are regular bursts of radiation on it – sometimes several times a day. “Proxima Centauri is as old as our sun, so it has probably been beating its planets with high-energy flares for billions of years,” explains co-author Alycia Weinberger of the Carnegie Institution for Science in Washington DC.
Observations by a team of astronomers led by Meredith MacGregor from the University of Colorado now show how violent the bursts of radiation can be at Proxima Centauri. The researchers targeted our neighboring star with nine earth-based telescopes stationed in orbit – including the Hubble and TESS space telescopes and the radio antennas of the Atacama Large Millimeter Array (ALMA). In spring and summer 2019, they kept pointing the optics of these telescopes at Proxima Centauri, in the hope of being able to capture a flare for the first time with so many different telescopes in parallel. On May 1, 2019, the time had come: “The star suddenly became 14,000 times brighter in ultraviolet light,” reports MacGregor. In the radio and millimeter wave range, the brightness of the red dwarf rose abruptly by a factor of a thousand. “In the past, we didn’t even know that stars also shine in the millimeter range when they erupt,” explains MacGregor. “This is the first time that we have observed a starburst in such a large spectral range.”
What was the cause?
The entire burst of radiation lasted less than ten seconds, but during this time it released large amounts of energy, especially in the UV and radio wave range. “In the millimeter and far UV range of the spectrum, this is the strongest flare ever observed by Proxima Centauri,” report the astronomers. In contrast, the telescopes registered only a slight increase in brightness in the visible range of light, while the TESS values only increased by around 0.9 percent. In addition, the radiation pulse in visible light only reached its peak with a one-minute delay. “These features have never been observed in such a dwarf star – that suggests that we are seeing a whole new kind of event here,” write MacGregor and her team. Typically, stellar bursts of rays occur when the magnetic field lines abruptly reconfigure themselves on the star’s surface. At Proxima Centauri, too, astronomers attribute the outbreak to the reconnection of magnetic field lines and the electrons accelerated as a result. The optical radiation emitted with a slight delay, on the other hand, could be due to the heated plasma at the foot of the magnetic loops – so the assumption.
For the two planets around our neighboring star, however, such extreme bursts of radiation are not good news. “If there were to be life on the inner planet of Proxima Centauri, then it would have to look very different from anything we know of the earth,” explains MacGregor. “A person would fare pretty badly on this planet.” In addition, such a violent outbreak of radiation as that registered on May 1, 2019 is by no means rare on red dwarfs like Proxima Centauri. “The planets around Proxima Centauri are hit by such flares not only once a century, but probably at least once a day, if not several times a day,” says MacGregor.
Source: Meredith MacGregor (University of Colorado, Boulder) det al., The Astrophysical Journal Letters, doi: 10.3847 / 2041-8213 / abf14c