Hubble Spots Infant Planets' Destruction Near Star

Astronomers have repeatedly observed events near a distant star that mirror processes from the early solar system's planet-forming era

To the point

• Powerful collisions observed in space: Hubble images show the formation of two enormous dust clouds caused by collisions between kilometre-sized celestial bodies in the young planetary system around the star Fomalhaut.
• A journey into the past of our solar system: Such events reflect processes that probably also took place in the early formation of our own solar system.
• Mysterious accumulation of rare events: Two such rare collisions within just 20 years call into question previous theories that assumed much longer intervals between such events.
• Significance for exoplanet research: The dust clouds can easily be confused with real planets - an important clue for future observation missions searching for exoplanets.

Astronomers, including MPIA's Bin Ren, have used the Hubble Space Telescope to capture the effects of catastrophic collisions between large rocky bodies in a nearby evolving planetary system around the star Fomalhaut. These events align with the widely accepted model in which planetesimals, asteroids, and comets collide, generating debris that contributes to planet formation. Scientists believe the early solar system underwent a similar period of intense activity, during which debris from these collisions later bombarded the young Earth, Moon, and other inner planets.

"This is certainly the first time I've ever seen a point of light appear out of nowhere in an exoplanetary system," said principal investigator Paul Kalas of the University of California, Berkeley, USA. "It's absent in all of our previous Hubble images, which means that we just witnessed a violent collision between two massive objects forming a huge debris cloud - unlike anything in our solar system today."

Just 25 light-years from Earth, Fomalhaut is one of the brightest stars in the night sky. Located in the constellation Piscis Austrinus, also known as the Southern Fish, it is more massive, brighter, and considerably younger than the Sun and hosts several belts of dusty debris.

MPIA (Max Planck Institute for Astronomy, Heidelberg, Germany) scientist and Hubble specialist Bin Ren, also affiliated with the Côte d'Azur Observatory in Nice, France, cross-checked that the signal was authentic by applying his own data processing algorithms. "In this way, we ensured that the 'detection' was not caused by some initial data preparation step that introduced fake signals," Bin Ren said.

In 2008, Fomalhaut was the first stellar system with a candidate extrasolar planet discovered in visible light. Found with Hubble, the object called Fomalhaut b later turned out to be an expanding dust cloud masquerading as a planet - the result of colliding planetesimals, kilometre-sized rocky building blocks of planets. While they intended to search for Fomalhaut b in recent Hubble observations, scientists now found a second point of light at a similar location instead. They call this object "cs2," while the first object is now known as "cs1."

Investigating the properties of planetesimal collisions

Why astronomers are seeing both these debris clouds so physically close to each other is a mystery. If the collisions between asteroids and planetesimals were random, cs1 and cs2 should appear at unrelated locations. Yet, they are positioned intriguingly near each other along the inner rim of Fomalhaut's outer debris disk.

Another mystery is why scientists have witnessed these two events within such a short timeframe. Theory suggests that there should be one collision every 100,000 years or longer. However, the astronomers have seen two within 20 years. In comparison, those events appear as a burst of fireworks. Indeed, such collisions are fundamental to the evolution of planetary systems, but they are rare and challenging to study.

"There may have been hundreds of such collisions that remained undetected," Ren pointed out. "Only those two were bright enough for Hubble to be detected. Fomalhaut's proximity helped discover those faint bursts of light in the first place."

The exciting aspect of this observation is that it allows researchers to estimate both the size of the colliding bodies and how many of them there are in the disk, information which is almost impossible to get by any other means. As a result, the planetesimals that were destroyed to create cs1 and cs2 measured 30 kilometres in size. There should be approximately 300 million such objects orbiting in the Fomalhaut system.

In 2022, NASA deliberately smashed a spacecraft into an asteroid during its first test mission for planetary defence, the Double Asteroid Redirection Test (DART). The impact ejected a spectacular dust cloud, but the scale of the Fomalhaut system collision is a billion times more violent. The Fomalhaut system is a natural laboratory to probe how planetesimals behave when undergoing collisions, which in turn tells astronomers about what they are made of and how they formed.

A lesson of caution for planet hunters

The transient nature of cs1 and cs2 poses challenges for future direct-imaging exoplanet missions, which may mistake such dust clouds for actual planets.

"Fomalhaut cs2 looks exactly like an extrasolar planet reflecting starlight," said Kalas. "What we learned from studying cs1 is that a large dust cloud can masquerade as a planet for many years. This is a cautionary note for future missions that aim to detect extrasolar planets in reflected light."

Looking into the future

The team has been granted additional observing time with Hubble to monitor cs2 over the next three years. The goal is to investigate how it evolves. Does it fade or get brighter? Being closer to the dust belt than cs1, the expanding cs2 cloud is more likely to start encountering other material in the belt. This could lead to a sudden avalanche of more dust in the system, which could cause the whole surrounding area to lighten up.

The team also proposed to use the NIRCam (Near-Infrared Camera) instrument on the James Webb Space Telescope to observe cs2. Webb's NIRCam will provide colour information that Hubble's STIS (Space Telescope Imaging Spectrograph) instrument could not. This colour data can reveal the size of the cloud's dust grains and their composition. It can even determine if the cloud contains water ice.

Hubble and Webb are the only observatories capable of this kind of imaging. While Hubble primarily sees visible wavelengths, Webb will capture cs2 in the infrared. These different, complementary wavelengths are needed to provide a broad multi-spectral investigation and a more complete picture of the mysterious Fomalhaut system and its evolution.

Background information

Bin Ren is the only MPIA astronomer who contributed to this research (also Observatoire de la Côte d'Azur, Nice, France).

Other researchers included Paul Kalas (Astronomy Department, University of California, Berkeley, USA; SETI Institute, Carl Sagan Center, Mountain View, USA; Institute of Astrophysics, FORTH, Heraklion, Greece), Jason J. Wang(Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, USA), and Maxwell A. Millar-Blanchaer (Department of Physics, University of California, Santa Barbara, USA).

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency).

This press release is largely based on the one published by STScI.

MN