It’s Asteroid Day!
Do planets in other star systems—and specifically those that could otherwise host some form of life—get frequently bombarded by asteroids?
The search for exoplanets has thus far concentrated on identifying those in so-called “habitable zones” where liquid water may exist on their surfaces. One is Proxima b in the closest star system to the Solar System—Proxima Centauri.
However, finding a potentially wet, warm planet is not much use if there’s an unchecked asteroid belt nearby that sends rocks to impact the planet and boil-off its oceans.
That’s the message behind new research published by Amir Siraj and Avi Loeb at Harvard University that considers the implications of an asteroid belt between the planets Proxima b and the recently discovered Proxima c.
They suggest that Proxima b could well have been already sterilized of life by large asteroid impacts.
Asteroid Day marks the anniversary of the Tunguska asteroid impact on June 30, 1908 that devastated a large part of Siberia.
The news about Proxima c underlines how important asteroid belts are in star systems, and in the hunt for other Earths.
Our solar system has an asteroid belt between Mars and Jupiter that’s the leftover debris of the planets’ formation. It’s hugely affected by the gravitational effects of Jupiter and Saturn; over time Earth has been both an open goal for asteroids, and protected from them.
Asteroids are both life-givers and life-takers.
“Small bodies likely delivered water to Earth, implying that some flux of impacts is a good thing, and might even have been necessary for life,” said co-author Amir Siraj at Harvard University, to me in an email. “However, large impacts can be devastating for life, an idea that hasn’t been studied closely outside of the context of Earth.”
How to find asteroid belts around other stars
Finding asteroid belts around other stars may be critical in the search for other Earths, but it’s not an easy task. “Asteroid belts around other stars have been detected, but typically at distances much further than the water ice line, which is roughly where the Solar System’s asteroid belt is located,” said Siraj.
Incredibly sensitive telescopes are needed to observe the ice lines in other star systems; this is challenging, but important work. “As the number of asteroid belts discovered around other stars grows, we will start to get a sense of the fraction of habitable planets that might have already been sterilized by a giant asteroid impact,” said Siraj.
“This would affect estimates of habitability and force us to reconsider the traditional definition of the ‘habitable zone’.”
Are Proxima b and Proxima c habitable?
Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Proxima c is likely too cold for life to exist there.
Asteroid impacts or not, life is unlikely on Proxima b simply because their parent stars—Proxima Centauri—is a red dwarf star that has a tendency to “flare.” That’s not good news for the possibility of life. “Proxima Centauri’s massive and horrifying flares already present a significant threat for the existence of life-as-we-know-it on Proxima b, as the ultraviolet radiation reaching the surface of the planet may be strong enough to kill some of the hardiest organisms on Earth,” said Siraj. “Living underwater could protect an organism from the harmful radiation, but if a sterilizing impact occurred on Proxima b, any oceans would have been boiled off.”
How do we know if Proxima b has ben ‘sterilized’?
In the study, the authors say that the chance that Earth was sterilized by asteroid impacts is less than 1%. So what is that figure for Proxima b? “Until an asteroid belt near Proxima b is definitely observed, we can’t say what the chance of sterilization is,” says Siraj.
In 2017 a detection of a belt at a distance of 0.4 AU from Proxima Centauri was reported, but later disputed as a possible stellar flare. “The discovery of an asteroid belt similar in mass to the Solar System’s at a distance of about 1/3 to 1/2 of an astronomical unit from Proxima Centauri would mean that Proxima b could have likely been sterilized already, with any oceans that it may have had evaporated as a result of the impact.”
Future observations of Proxima Centauri by ALMA—which measures millimeter-wavelength emission—and the James Webb Space Telescope (JWST)—which measures infrared-wavelength emission—will reveal if there actually exists an asteroid belt near Proxima b. “Both millimeter-wavelength and infrared-wavelength emission which would be produced by debris in an asteroid belt,” said Siraj. “They both have excellent angular resolution capabilities, making them ideal for searching for asteroid belts.”
“JWST should be capable of characterizing asteroid belts around some of the nearest stars,” said Siraj. “This would come with the immense reward of better understanding the chances for life on potentially habitable planets.”
Wishing you clear skies and wide eyes.