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Planet hunters discover huge super-Jupiter by watching nearby star wobble


The Beta Pictoris b isn't so lonely anymore.

ESO / L. Calçada / N. Risinger

For the last decade, astronomers on Earth have thought of Beta Pictoris b as a lonely exoplanet orbiting within a huge, dusty disk around the Beta Pictoris star. The planet, a super-Jupiter about 13 times as massive as our Jupiter, was the only known planet in the young planetary system approximately 63 light-years from Earth. That was, until today.

New research, published in the journal Nature Astronomy on Monday, reveals another planet in the Beta Pictoris system, Beta Pictoris c. The discovery of the planet, another super-Jupiter with approximately nine times the mass of Jupiter, throws a cosmic spanner into our current understanding of the well-studied but largely mysterious Beta Pictoris system. The system is somewhat famous in astronomy circles because it is only 23 million years old and surrounded by a rock and ice disk, allowing researchers a way to study how planetary systems form and how they form.

Beta Pictoris c was discovered by the same team responsible for discovering Beta Pictoris b over a decade ago, led by French astronomer Anne-Marie Lagrange who has been studying the system for more than 30 years. This time, the hunt was a little more indirect.

To search for planets outside our solar system, astronomers use a host of different methods. One of the least favorable for finding planets is known as "direct imaging". The method can detect a planet by seeing the light it reflects from its host star, but most of the time the host star is so bright we can't make out the faint light it gives off. Fortunately for Lagrange and her colleagues, Beta Pictoris b, which is about the same distance from its star as Saturn is from our sun, was found after meticulously studying data from Chile's Very Large Telescope.

A "radial velocity" measurement looks at the host star for minor disturbances in its orbit caused by other huge cosmic bodies. Astronomers can measure these disturbances and determine what causes them by measuring the light that reaches the Earth. In any given year, radial velocity measurements detect dozens of exoplanets, but they're still nowhere near as effective at distant distant worlds as NASA's Kepler or TESS telescope.

Fortunately, Beta Pictoris c was able to find this way. It just wasn't easy.

"We monitored the radial velocities of Beta Pictoris (which is really complicated because the star rotates so rapidly), and analyzed the data (which was also very complicated because the star pulsates)," Lagrange said, via email.

A specialized instrument installed at Chile's La Silla Observatory, the High Accuracy Radial Velocity Planet Searcher (HARPS), provided 15 years of data to help pinpoint the new planet. The team says the Beta Pictoris c lies approximately 2.7 AU from its star. For comparison, that puts it in about the same position as the solar system's asteroid belt.

If the signal they detect is another planet it would mark the first time a multi-planet system has been found using direct imaging and indirect detection methods. Lagrange says the discovery shows planetary systems can be extremely massive, with this one now featuring approximately 20 times the mass of Jupiter in just two worlds. And if there are other smaller planets in the system, the massive Beta Pictoris c is likely to influence their orbits, too.

Lagrange says she will continue to study Beta Pictoris and search for additional planets, which could help explain some of the more challenging questions posed by her latest discovery. It is hypothesized that Beta Pictoris c may also transmit in front of the star (as seen from Earth), which would allow the team to characterize its size, atmosphere and discover whether it has any moons or rings.

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