Astronomers have achieved something new: They can now measure the orbit of an exoplanet by measuring its motion around the star. This is something that they have never done before. The gravity of the second planet pulls on the first and changes its orbit slightly around its star. This effect can be directly measured.
This impressive work shows how advanced our techniques are for looking at other planet systems.
The star Beta Pictoris is about 63 light-years from Earth. It orbits the planets. It is visible from the southern latitudes, so it is easily seen by the naked eye. It was discovered that Beta Pic, as it is affectionately called, had too much infrared light coming out of it in the 1980s. This suggests it could be orbited and warmed by a disk made up of dust. In the 1990s, images of the disk were made directly. As time passed by, the observations have improved.
The shape of the disk indicated the presence of a world. The inner edge of the disk was bent, which indicates a planet orbiting near the star on an inclined orbit. Because of the gravity of the planet, the disk was slightly bent.
In 2008, astronomers reported that they had discovered Beta Pictoris B. This planet has 9 times the mass as Jupiter and orbits the star 1.5 billion km out. It is about the same distance as a Saturn orbits the Sun. The orbital period is the time it takes to orbit once. It is 23 years.
Direct imaging was used to discover the planet. This means that they could actually see it in star images! Observations over time have shown the planet's motion as it orbits the star almost as closely as from Earth.
Beta Pictoris' apparent motion is due to its space velocity and changing perspective as Earth orbits Sun. It becomes a corkscrew, but it is also thrown off by Beta Pic b. Credit: ESA Photo: ESA
In 2019, Beta Pictoris C, a second planet was discovered. It orbits closer to the star (about 400mi from the Sun's asteroid belt) and is large enough that its gravity pulls Beta Pic in a small circle as it moves around the star. Although it is difficult to measure, the Doppler shift can detect this reflex motion. Beta Pic c was confirmed to exist in images taken of this system a year later. It is also massive at 9 to 10 times Jupiter's weight and orbits for about 3.3 years.
This is where the new work begins.
Schematic showing Beta Pic b, and c relative to the star, and disk. Based on actual observations in 2020. Credit to GRAVITY Collaboration/ Axel M. Quetz.
The path of a planet around a star will be an ellipse if it orbits the star alone. However, if a second planet is near, it will pull on the first planet gravityally and the orbit will diverge from an ellipse. You can theoretically take lots of observations of the planet and meticulously measure its position each time. Then calculate its orbit. You're done if it's an "ellipse". If it displays periodic perturbation, you are on to something.
This is what a team made up of astronomers. They used archives of Beta Pic B observations from the past, and then added new observations with the GRAVITY instrument on the 8.2-meter telescope that makes up the Very Large Telescope in Chile. The orbit of the planet was first calculated, assuming that the planet orbits the star alone. Next, they also assumed that a second planet exists. Finally, the calculations determined the best fit for this second planet (which is how much mass and what orbit it must have to best explain the positions for planet b).
They saw it in their data! The mass was a little higher than the previous measurements. It was also larger than expected. Although they weren't able restrict the orbit of the object much, it was detectable.
We're not talking about tiny deviations in a motion already so small in the sky. Imagine that you are in Los Angeles, and that you have rolled a marble across the floor. A friend is in New York City and observes Beta Pic c. Your friend will see your marble move by one centimeter. One centimeter. One continent away.
Illustration of the star Beta Pictoris with its dust disk. Credit: GRAVITY Collaboration / Axel M. Quetz, MPIA Graphics Department
Astronomers could do better. The accuracy of their orbital calculations is improved by adding observations of planet C to the equations. They were able to calculate a mass of 9.15 times Jupiters (+-1.08/1.06) which was closer than previous estimates. The orbital shape was also quite elliptical with an eccentricity value of 0.3. An eccentricity of 1 or 0 means that it is a line and a circle.
They also included the reflex velocity information for planet c, which further improved the mass and orbit. They found that planet c is 8.89 + 0.75 Jupiters in mass, has an orbital axis of 402 million kilometers and a period of 1221 days. It is remarkable that all of these numbers agree within the uncertainty.
Although Beta Pic c was known to exist before they discovered it, that doesn't mean it didn't happen. It was only discovered by them looking at planet b. However, they were able to refine their calculations using prior knowledge of planet.
Astronomers have been searching for answers to a long-standing question. It's unclear why planet b orbits in an eccentric way. Is it possible that it formed this way or was it caused by something else? Planet c is large enough and close enough to planet B that they can interact over the course of time. The outer planet could cause the orbit of the inner one to become an ellipse. Although it is not proven, this is at least possible and consistent with data. The existence of the planet could have an impact on material in the disk orbiting a star, which would add to the complexity of this system's interactions.
When the disk around Beta Pic was first discovered, I was just beginning my undergraduate studies in astronomy. I kept an eye out for it over the years and even built a camera to take images of it in the 2000s. Although I haven't studied the system or worked on its data, it has been fascinating to watch our understanding grow and my own knowledge of astronomy.
We're not done with Beta Pictoris. We still have a lot to learn about this complex system of planets.
Jason Wang, who was part of the team that performed this work, deserves a tip o' dew shield