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In 2008, astronomers made a big announcement: A giant exoplanet had been visually seen in Hubble images of the nearby bright star Fomalhaut! It was very exciting news; Most of the planets at that point (and even today) are detected indirectly, so physically seeing one in a picture was big business.
Furthermore, Fomalhaut is only 25 light years away, one of the brightest stars in the sky, easily visible to the naked eye, and very young, a few hundred million years old. It is also surrounded by a huge ring of dust that makes it very much like the Eye of Sauron, which many people (including me) had a lot of fun with.
It was even observed that the planet, called Fomalhaut b, moved in observations between 2004 and 2010, confirming that it was a real object.
… but then, problems. Observations using the Spitzer Space Telescope were distressingly blank, when the planet should have been bright infrared. But the images showed nothing. There’s definitely something there in the Hubble footage, but now nobody was sure what. Perhaps a planet surrounded by a cloud of dust, perhaps due to colliding moons?
Now, a new article has been published by a couple of astronomers who make a claim that is equal parts. journal and even cooler: Not a planet at all (bummer); It’s the expanding dust of two large asteroids that collided at high speed!
Even cooler.
To be clear, this is still a hypothesis, an idea, but your analysis of the Hubble images shows that the observations are consistent with an event like this. And it’s also compelling.
The idea is based on three conclusions from his analysis: 1) The object is moving out of the star system, 2) it is getting bigger, and 3) it is fading.
By observing Hubble data taken from 2004 to 2014 using the Advanced Survey Camera and the Space Telescope Imaging Spectrograph, they were able to measure the position of the object over time. From that, an orbit can be calculated, and they discover that it appears to be moving fast enough to escape Fomalhaut. That would be highly unlikely for a planet, but the remains of two asteroids may collide. Many of the smaller dust grains generated in such an impact would feel an external force due to the bright light from Fomalhaut (which is larger, more massive, and approximately 16 times brighter than the Sun). In a few years they could feel enough acceleration of this force to unleash, achieving escape velocity.
By looking at the data over time, you can also see that the object is an unresolved point in the first images, but clearly stretches out in the subsequent ones, indicating that it is expanding. That’s what you would expect from a debris cloud from a collision. Also, it gets weaker over time, which again makes sense. Once a debris cloud expands to a certain size, the density drop causes it to fade; it scatters too much to capture starlight and reflect it efficiently.
This idea is quite provocative. But does it make physical sense? Can two asteroids collide with each other strong enough to make a cloud like this?
According to the models that the authors ran, yes. It would take two very large rocks, certainly greater than 100 km and more likely up to 200 km (or one smaller if the other is much larger), colliding at speeds of 200–600 meters per second (~ 700 to 2200 kph) to generate so much dust. Those relative speeds are completely reasonable for asteroids at that distance from the star (approximately 15 billion km, approximately 3 times the distance from Neptune from the Sun).
Therefore, his idea is internally consistent and also appears to be consistent with the observations. My biggest problem with this is that such large events are rare: the authors calculate that it happens about once every 200,000 years in Fomalhaut, and it had to happen not long before the first observations in 2004. Scientists generally pause when you need allege special circumstances; we prefer general cases. But this case really looks special. The fading and expansion rates really indicate that it must have happened very recently, and we were lucky. It is certainly not impossible, but it is always good to maintain a reasonable level of skepticism.
What follows then? The next obvious step is to get more observations. That is unlikely for Hubble; The last set of observations taken in 2014 showed that the object had vanished into invisibility. But the James Webb Space Telescope will launch soon (for broad enough definitions of “soon”) and they have observations scheduled for the first year that the telescope is operational! That’s exciting. Webb should be powerful enough to detect it and hopefully clear this up.
This is the nature of pushing observations to the limit. We don’t always know what we will get, even after we have received it! Identifying Fomalhaut b as a planet, then not, then back again, then something else, and now perhaps a cloud of debris, is quite normal for the course. Something like this happens a lot when we build new observatories and push them as hard as they can be pushed in a field (such as exoplanet exploration by direct imaging) that is just beginning.
There is a reason why it is called the avant-garde. And there is a reason why we call it exploration. If we knew what we would get, it wouldn’t be.
