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Planet formation in action

Astronomers may have found the first observable object clearing its path in the natal disk surrounding a young star.
Using the European Southern Observatory’s (ESO) Very Large Telescope (VLT), an international team of astronomers has been able to study the short-lived disk of material around a young star that is in the early stages of making a planetary system. For the first time, a smaller companion could be detected that may be the cause of the large gap found in the disk. Future observations will determine whether this companion is a planet or a brown dwarf.

Planets form from the disks of material around young stars, but the transition from dust disk to planetary system is rapid and few objects are caught during this phase. One such object is T Chamaeleontis (T Cha), a faint star comparable to the Sun in the small southern constellation Chamaeleon that’s near the beginning of its life. T Cha lies about 350 light-years from Earth and is only about 7 million years old. Up to now, no forming planets have been found in these transitional disks, although planets in more mature disks have been seen before.

"Earlier studies had shown that T Cha was an excellent target for studying how planetary systems form," said Johan Olofsson from the Max Planck Institute for Astronomy in Heidelberg, Germany. "But this star is quite distant, and the full power of the Very Large Telescope Interferometer (VLTI) was needed to resolve very fine details and see what is going on in the dust disk."

The astronomers first observed T Cha using the AMBER instrument and the VLTI. They found that some of the disk material formed a narrow dusty ring only about 12 million miles (20 million kilometers) from the star. Beyond this inner disk, they found a region devoid of dust with the outer part of the disk stretching out into regions beyond about 700 million miles (1.1 billion kilometers) from the star.

"For us, the gap in the dust disk around T Cha was a smoking gun, and we asked ourselves if we could be witnessing a companion digging a gap inside its protoplanetary disk,” said Nuria Huelamo from Centro de Astrobiologia, ESAC, Spain.

However, finding a faint companion so close to a bright star is a huge challenge, and the team had to use the VLT instrument NACO in a novel and powerful way, called sparse aperture masking, to reach their goal. After careful analysis, they found the clear signature of an object located within the gap in the dust disk about 600 million miles (1 billion kilometers) from the star — slightly further out than Jupiter is within our solar system and close to the outer edge of the gap. This is the first detection of an object much smaller than a star within a gap in the planet-forming dust disk around a young star. The evidence suggests that the companion object cannot be a normal star, but it could be either a brown dwarf surrounded by dust or a recently formed planet.

"This is a remarkable joint study that combines two different state-of-the-art instruments at ESO's Paranal Observatory,” said Huelamo. “Future observations will allow us to find out more about the companion and the disk, and also understand what fuels the inner dusty disk."

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