Scientists compute exoplanet’s mass using star-mapping satellite data

Hipparcos and Gaia measurements help scientists weigh extremely young exoplanet.

Using data collected by the European Space Agency’s (ESA) star-mapping satellites Hipparcos and Gaia, a team of researchers successfully calculated the mass of a baby exoplanet discovered in 2008.

Beta Pictoris b, a gas giant discovered by the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile, is a gas giant of 9-13 Jupiter masses orbiting the second brightest star in the constellation Pictor.

Hipparcos observed the Beta Pictoris system 111 times between 1990 and 1993. Gaia, launched in 2013, imaged the system 30 times since it began operating 22 months ago with the goal of looking at more than one billion stars in the Milky Way.

Beta Pictoris and its planet are approximately 20 million years old, making them 225 times younger than our solar system. Such young systems can be difficult to study because their stars are very hot, pulsate, and rotate very fast.

“In the Beta Pictoris system, the planet has essentially just formed. Therefore, we can get a picture of how planets form and how they behave in the early stages of their evolution,” said Ignas Snellen of Leiden University in the Netherlands.

Scientists typically measure a star’s radial velocity, the speed at which it regularly moves toward and away from Earth, to estimate the masses of planet(s) orbiting it. But this method works largely for older systems, where planet formation is complete.

An upper limit for Beta Pictoris b’s mass was established through the radial velocity method. Snellen and Anthony Brown, also of Leiden University, then studied measurements of the system taken by Hipparcos and Gaia, which gave them the exact position of the star as well as its motions 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,” Snellen explained.

Tiny wobbles in a star’s path are caused by the gravitational tugs of orbiting planets.

“We are looking at the deviation from what you would 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,” Brown stated.

Doing this requires observation of a star over long periods of time, he added.

“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.”

A paper on the study’s findings has been published in the journal Nature Astronomy.


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