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SYFY WIRE Bad Astronomy

Vacuuming up the dust — and sterilizing a planet — around Proxima Centauri

By Phil Plait
Artwork depicting a flare from a low-mass star. Credit: Casey Reed/ NASA

Well, nuts.

Last week, just five days ago, I wrote that astronomers had found evidence of rings of dust around Proxima Centauri, the nearest star to our own, and furthermore that there might be a ringed planet there.

As I was literally drafting up that article, a different team of astronomers — using the very same data — were putting the final touches on their own paper showing, pretty convincingly, that all those conclusions are very likely wrong.

Maybe even worse, they show that Proxima Centauri b, the roughly Earth-sized planet orbiting the star, has probably been torched by the huge flares the star pops off.

Like I said: Nuts. But we need to see why this story evolved, so let's dive into this.

Alpha and Proxima Centauri

The observations were made using the Atacama Millimeter/submillimeter Large Array, or ALMA. It observes light with far longer wavelengths than the light we see; out in the Universe this is the sort of light emitted by objects not much warmer than absolute zero. In general that means things like gas and dust clouds.

The first team of astronomers observed Proxima Centauri, a low-mass, small, and very dim red dwarf. Despite being less than 4.3 light years away, and therefore the closest star to the Sun, it's so faint that it can be tricky to observe. Still, very careful observations have found a planet orbiting the star; called Proxima Centauri b (or just Proxima b for short) it has a minimum mass of 1.3 times Earth's, meaning it's close to our own planet in mass, and orbits Proxima in the habitable zone, where the temperature is in the range where liquid water could exist on the planet's surface.

These astronomers found some interesting things. The star itself is unresolved in their observations, meaning it's just a dot. But it was brighter in their observations than would be expected for a red dwarf. They posited this could be explained by a ring of dust orbiting the star between 150 and 600 million kilometers out. That would have the right properties to explain the extra emission.

They also note there could be an inner ring of dust just 60 million km out, and several blobs of material could trace be a third, outer ring of dust 500 billion km out. Finally, some excess emission close to the star might be explained by a planet with rings, much like Saturn. This is all very exciting.

That's an exciting prospect!

Except, alas, it looks like it's not to be. A second team of astronomers took a look at the data as well, and processed it in a different way. The first team simply added together all the observations, building up a static picture of what the system looks like. The second team looked at the individual observations, which were taken over a time span of several months, from January to April 2017.

In observations made on March 24, 2017, they found a surprise: The star got much brighter very suddenly on that day. And by much brighter, I mean nearly a thousand times its usual brightness! This surge lasted only about a minute, after which the star calmed down.

The obvious explanation is a stellar flare, what we call a solar flare when it occurs on the Sun. These are caused when magnetic field lines piercing the surface of the star get tangled up, snap, and release their energy. On the Sun these can be extremely energetic, equaling 10% of the Sun's total output of energy! In terrestrial terms, that's like detonating 10 billion one-megaton nukes all at the same time.

So yeah. Wow.

Artwork of a red dwarf in a binary system undergoing a flare. Credit: NASA's Goddard Space Flight Center/S. Wiessinger

And the thing is, stars like Proxima are known to be very active magnetically. This happens because even though these stars are low mass and dim, they have very strong and chaotic magnetic fields. These fields are generated inside the star by convecting (rising and falling) parcels of ionized gas. This material starts just outside the star's core and reaches all the way to the surface, so the magnetic field can be far stronger than ones for much bigger stars.

Because of that, Proxima and stars like it flare pretty often, and some of these explosions can be mind-crushingly violent. X-ray flares (the most energetic kind seen) up to 10 times as bright as the star's normal (quiescent) output happen every few days, and less energetic visible light ones several times a day.

And this, argues the second team of astronomers, explains the observations. The first team added all the observations together, so they found the star to be too bright (which they explain by dust adding to its brightness), but that's actually due to the flare near the end of their observations. Take that out, and the star looks quite naked, devoid of dust.

There were several smaller flares seen right before the big one too, and those can explain what the first team thought might be an inner ring of dust. The emission from those smaller flares looks just like what you'd expect from a warm ring of dust, if you don't look at each observation individually.

Adding all the ALMA observations together (left) indicates an elongation around Proxima Centauri to the lower left. But when you look at the individual observations, it's only seen in one (right) and not the other (middle). Credit: MacGregor et al.

Even the elongation of the central source, which the first team supposes could be a planet, is explained by looking at the observations individually. It's seen in one set but not the other, indicating it's probably just noise, random blips in the data.

Finally, that outer ring? They find that the number of background sources in the field could explain them too. Proxima lies very near the plane of the Milky Way galaxy as seen from Earth, so there could be lots of thin cold gas (called galactic cirrus) in the background that could look like blobs in the data. It's hard to say for sure, and more observations are needed.

So all of this is a bummer; rings of dust around the star was an exciting prospect (if only because they gave us info about the geometry of the system).

But it gets worse.

When they calculate the total energy of the big flare, it is ten times more powerful than one of the Sun's bigger flares! That's a lot of energy. So much, in fact, that the planet, Proxima b, would get good and fried by it. I mean crispy. The planet orbits the star much closer than Earth does the Sun, about 7 million kilometers from Proxima, so the energy from the flare would hit it a lot harder. Assuming these flares happen relatively often (a very safe bet), over the lifetime of the planet these would basically sandblast the planet, ripping the atmosphere right off the planet. They'd strip away any oceans, too, and sterilize whatever was left.

It's hard to overstate the damage. These flares, over billions of years, are downright apocalyptic. It's hard to imagine anything being able to survive. The planet may very well be a completely zapped airless lifeless ball of rock.


But that's science. Actually, that's two very important aspects of science: One is that sometimes getting new data or reassessing old data gives very different results, and the old conclusions need to be discarded (a corollary: Science is done by people, and we all make mistakes; the trick is to learn from them). The other is that even if the results are a massive bummer, welp, that's reality. It's under no obligation to be all unicorns and rainbows all the time.

The good news is that we're learning more about Proxima, and also that the existence of the planet itself isn't really in doubt. It's there. So that's still cool.

Science takes away, but it also gives. If you find comfort in that, wonderful! As for me, I'd rather see the world, the Universe, the way they truly are. For better or for worse.