Credits: ESO and S. Haffert (Leiden Observatory)
Astronomers have directly imaged two exoplanets that are
gravitationally carving out a wide gap within a planet-forming disk
surrounding a young star. While over a dozen exoplanets have been
directly imaged, this is only the second multi-planet system to be
photographed. (The first was a four-planet system orbiting the star HR 8799.) Unlike HR 8799, though, the planets in this system are still growing by accreting material from the disk.
“This is the first unambiguous detection of a two-planet system
carving a disk gap,” said Julien Girard of the Space Telescope Science
Institute in Baltimore, Maryland.
The host star, known as PDS 70, is located about 370 light-years from
Earth. The young 6-million-year-old star is slightly smaller and less
massive than our Sun, and is still accreting gas. It is surrounded by a
disk of gas and dust that has a large gap extending from about 1.9 to
3.8 billion miles.
PDS 70 b, the innermost known planet, is located within the disk gap
at a distance of about 2 billion miles from its star, similar to the
orbit of Uranus in our solar system. The team estimates that it weighs
anywhere from 4 to 17 times as much as Jupiter. It was first detected in 2018.
PDS 70 c, the newly discovered planet, is located near the outer edge
of the disk gap at about 3.3 billion miles from the star, similar to
Neptune’s distance from our Sun. It is less massive than planet b,
weighing between 1 and 10 times as much as Jupiter. The two planetary
orbits are near a 2-to-1 resonance, meaning that the inner planet
circles the star twice in the time it takes the outer planet to go
around once.
The discovery of these two worlds is significant because it provides
direct evidence that forming planets can sweep enough material out of a
protoplanetary disk to create an observable gap.
“With facilities like ALMA, Hubble, or large ground-based optical telescopes with adaptive optics we see disks with rings and gaps all over. The open question has been, are there planets there? In this case, the answer is yes,” explained Girard.
The team detected PDS 70 c from the ground, using the MUSE spectrograph
on the European Southern Observatory’s Very Large Telescope (VLT).
Their new technique relied on the combination of the high spatial
resolution provided by the 8-meter telescope equipped with four lasers
and the instrument’s medium spectral resolution that allows it to “lock
onto” light emitted by hydrogen, which is a sign of gas accretion.
“This new observing mode
was developed to study galaxies and star clusters at higher spatial
resolution. But this new mode also makes it suitable for exoplanet
imaging, which was not the original science driver for the MUSE
instrument,” said Sebastiaan Haffert of Leiden Observatory, lead author
on the paper.
“We were very surprised when we found the second planet,” Haffert added.
In the future, NASA’s James Webb Space Telescope may be able to study
this system and other planet nurseries using a similar spectral
technique to narrow in on various wavelengths of light from hydrogen.
This would allow scientists to measure the temperature and density of
gas within the disk, which would help our understanding of the growth of
gas giant planets. The system might also be targeted by the WFIRST
mission, which will carry a high-performance coronagraph technology
demonstration that can block out the star’s light to reveal fainter
light from the surrounding disk and companion planets.
These results were published in the June 3 issue of Nature Astronomy.
Source: HubbleSite/News
