A duo of astronomers at Leiden University in the Netherlands has deduced the mass of Beta Pictoris b from the motion of its parent star, which lies 63 light-years from Earth and is estimated to be only 20 million years old, over a long period of time as captured by ESA’s Gaia satellite and its predecessor, the Hipparcos satellite. Beta Pictoris b is a gas giant similar to Jupiter but, according to the new estimate, is 9 to 13 times more massive.
This artist’s impression shows how the gas giant exoplanet inside the disc of Beta Pictoris may look. Image credit: L. Calçada / ESO.
Beta Pictoris b was discovered in November 2008 by Dr Anne-Marie Lagrange of the University Joseph Fourier and co-authors in images captured by ESO’s Very Large Telescope.
The planet orbits its host star, Beta Pictoris, at a distance of 8 times the Earth-Sun distance.
The star also hosts a circumstellar disk of gas and dust that could, in time, evolve into a torus of icy bodies much like Solar System’s Kuiper Belt.
“In the Beta Pictoris system, the planet has essentially just formed,” said Leiden University’s Professor Ignas Snellen.
“Therefore we can get a picture of how planets form and how they behave in the early stages of their evolution. On the other hand, the star is very hot, rotates fast, and it pulsates.”
This behavior makes it difficult for astronomers to accurately measure the star’s radial velocity — the speed at which it appears to periodically move towards and away from the Earth.
Tiny changes in the radial velocity of a star, caused by the gravitational pull of planets in its vicinity, are commonly used to estimate masses of exoplanets. But this method mainly works for systems that have already gone through the fiery early stages of their evolution.
In the case of Beta Pictoris b, upper limits of the planet’s mass range had been arrived at before using the radial velocity method.
To obtain a better estimate, Professor Snellen and Leiden University astronomer Dr. Anthony Brown used a different method, taking advantage of Hipparcos’ and Gaia’s measurements that reveal the precise position and motion of the planet’s host star in the sky over time.
“The star moves for different reasons. First, the star circles around the center of the Milky Way, just as the Sun does. That appears from the Earth as a linear motion projected on the sky. We call it proper motion. And then there is the parallax effect, which is caused by the Earth orbiting around the Sun. Because of this, over the year, we see the star from slightly different angles,” Professor Snellen explained.
And then there is something that the astronomers describe as ‘tiny wobbles’ in the trajectory of the star across the sky — minuscule deviations from the expected course caused by the gravitational pull of the planet in the star’s orbit.
This is the same wobble that can be measured via changes in the radial velocity, but along a different direction — on the plane of the sky, rather than along the line of sight.
“We are looking at the deviation from what you expect if there was no planet and then we measure the mass of the planet from the significance of this deviation. The more massive the planet, the more significant the deviation,” Dr. Brown said.
To be able to make such an assessment, astronomers need to observe the trajectory of the star for a long period of time to properly understand the proper motion and the parallax effect.
The Gaia mission, designed to observe more than one billion stars in our Galaxy, will eventually be able to provide information about a large amount of exoplanets.
In the 22 months of observations included in Gaia’s second data release, the satellite has recorded the star Beta Pictoris about thirty times. That, however, is not enough.
Combining the Gaia measurements with those from ESA’s Hipparcos mission, which observed Beta Pictoris 111 times between 1990 and 1993, enabled the astronomers to get their result much faster.
This led to the first successful estimate of a young planet’s mass using astrometric measurements.
“By combining data from Hipparcos and Gaia, which have a time difference of about 25 years, you get a very long term proper motion,” Dr. Brown said.
“This proper motion also contains the component caused by the orbiting planet. Hipparcos on its own would not have been able to find this planet because it would look like a perfectly normal single star unless we had measured it for a much longer time.”
“Now, by combining Gaia and Hipparcos and looking at the difference in the long term and the short term proper motion, we can see the effect of the planet on the star.”
The findings were published in the journal Nature Astronomy.
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I.A.G. Snellen & A.G.A. Brown. The mass of the young planet Beta Pictoris b through the astrometric motion of its host star. Nature Astronomy, published online August 20, 2018; doi: 10.1038/s41550-018-0561-6
