Two baby gas giants spotted in orbit around a distant star

This artist’s illustration shows two gas giant exoplanets orbiting the young star PDS 70. J. Olmsted, STScI.

Two baby planets have been spotted forming around a distant young star called PDS 70. The two planets are growing into gas giants, and one of them came as a complete surprise to the scientists imaging the star.

PDS 70 is located 370 light-years away, and is a K7-type pre-main sequence star. That means it’s just a little bit smaller and less massive than our Sun, but is a relative infant by star standards at just 5.4 million years old.

When astronomers used the European Southern Observatory’s (ESO) Very Large Telescope to look at PDS 70, they were expecting to see one planet called PDS 70b. This baby planet is about 21 AU away from its star (one AU, or astronomical unit, is the distance between the Earth and the Sun) and is more massive than Jupiter by a factor of between four and 17 times.

The surprise was finding a second developing planet, called PDS 70c. This one is further out, at 34.5 AU from its star, and is smaller than its brother. Its mass is between one and ten times that of Jupiter.

PDS 70 is only the second multi-planet system to be directly imaged. Through a combination of adaptive optics and data processing, Haffert et al were able to cancel out the light from the central star (marked by a white star) to reveal two orbiting protoplanets, PDS 70 b (lower left) and PDS 70 c. ESO / S. Haffert, Leiden Observatory.

Rather neatly, the two planets are very close to what is called a “2:1 resonance,” meaning that PDS 70b travels around the star twice in the time it takes PDS 70c to travel around it once. The two planets are having an important effect on the disk of dust and gas around the star, carving out a gap where they pass through the material.

“This is the first unambiguous detection of a two-planet system carving a disk gap,” Dr. Julien Girard, an astronomer with the Space Telescope Science Institute, said in a statement.

Observing young planets is helpful because it can teach us about how stars and planets evolve over time. Scientists believe that gas giants like Saturn and Jupiter (and the two planets around PDS 70) are “failed stars” because they are formed from clumps of hydrogen gas, like stars, but do not become large or hot enough to begin fusion. This latest study shows how forming planets can eat through dust and gas in the disk around a star to fuel their growth.

The study is published in the journal Nature Astronomy.

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