63.4 light years from Earth in the constellation Pictor is the young and light blue star Beta Pictoris. In 2008, observations from ESO's Paranal Observatory in Chile confirmed the presence of an extrasolar planet. This planet was Beta Pictoris b, a Super Jupiter with an orbital period of up to 6890 to 8890 days (~ 19 to 24 years), which was confirmed by direct imaging behind the star.
In August 2019, a second planet (another Super Jupiter) was discovered orbiting closer to Beta Pictoris. Due to its proximity to the parent star, however, it could only be investigated indirectly (radial velocity measurements). After a new analysis of the data received from the VLT, astronomers, in collaboration with GRAVITY, were able to confirm the existence of Beta Pictoris c through direct imaging.
The researchers responsible for the research detailed their results in two studies published in the October 2nd issue of Astronomy & Astrophysics. The first was led by Mathias Nowack from the Kavli Institute of Cosmology (Cambridge University) while the second was led by Anne-Marie Lagrange from the Laboratory for Space Studies and Instrumentation in Astrophysics (LESIA) and the Paris Observatory.
An artistic illustration of Beta Pictoris b, the super Jupiter discovered using the direct imaging method. Credit: ESO / L. Calçada
As they explain in their first study, the GRAVITY collaboration combined light from four of the VLT's large telescopes to make the first direct observations of Beta Pictoris c. Not only was this the first time a planet was imaged this close to its parent star, but it was also the first time direct imaging was used to confirm a detection made with the radial velocity method (also known as Doppler spectroscopy) was carried out.
For decades, astronomers have relied on radial velocity measurements to determine the presence of exoplanets. With hundreds of candidates discovered so far, it has been the second most popular method (after the transit method) and is often combined with transits to confirm the existence of exoplanets. Never before have candidates discovered using the radial velocity method been confirmed by direct observation.
This was only possible with the GRAVITY instrument, a second generation device that is part of the VLT Interferometer (VLTI). This instrument combines light from four VLT telescopes – either the four unit telescopes or the four auxiliary telescopes each supported by Adaptive Optics (AO) – and then combines them into a virtual telescope, enabling unprecedented detail and sensitivity.
As Frank Eisenhauer, lead scientist of the GRAVITY project at the Max Planck Institute for Extraterrestrial Physics (MPE), recently said in an institute statement:
“It is amazing what level of detail and what sensitivity we can achieve with GRAVITY. We are only just beginning to explore stunning new worlds, from the supermassive black hole in the center of our galaxy to planets outside the solar system. "
A 2010 Perseid meteor brushes over the Very Large Telescope (VLT) of the European Southern Observatory. Photo credit: ESO
At the same time, the Beta Pictoris c team was only able to observe because new radial velocity measurements could accurately determine the orbital motion of the planet. This was the subject of the second paper, which showed how a combination of high contrast imaging, long baseline interferometry and radial velocity data allowed them to accurately predict the location of Beta Pictoris b so that GRAVITY could find it.
Their observations also allowed them to further constrain the orbital and physical properties of Beta Pictoris b and c and to predict the closest approximations of both planets. What this revealed, however, was quite a mystery to the GRAVITY team. Beta Pictoris c is about eight times as massive as Jupiter and about 2.7 AU from its star – about as far as the main asteroid belt from the Sun.
Yet c is about six times weaker than Beta Pictoris b, which orbits its star at a distance of about 9.8 AU, which is the distance between Saturn and the Sun. This begs the question of how big and massive it has to be for it to reflect six times as much light. Currently, the mass estimates for b are less limited, ranging from 10 to 11 Jupiter's masses.
However, as they indicate, future observations using the radial velocity method can answer this question. The only problem is that it will take several years as it takes Beta Pictoris b approximately 28 years to complete a single orbit around its star. Additional data could also be provided by GRAVITY +, the next generation tool currently under development.
Artist's impression of the path of the star S2, which the GRAVITY collaboration followed with the help of the VLT at the Paranal Observatory in Chile. Photo credit: ESO / M. Kornmesser
Paul Molliere, MPIA Postdoc modeling exoplanet spectra added:
“We previously used GRAVITY to obtain spectra of other directly imaged exoplanets that already contained clues about their formation process. This brightness measurement of Beta Pictoris c in combination with its mass is a particularly important step in limiting our planet formation models. "
The team's results also enabled them to limit the presence of additional planets in the Beta Pictor system. Thanks to the combined measurements and data, they can rule out the presence of planets more than 2.5 times as massive as Jupiter with 3 AU of the star. Planets with more than 3.5 Jupiter masses between 3 and 7.5 AU and more than 1-2 Jupiter masses over 7.5 AU.
Perhaps the most impressive lesson from this research, however, is how astronomers can now combine the greatest benefits of direct and indirect methods for exoplanet studies. As they explained in the first study, direct detection is more sensitive to planets orbiting a great distance from their host star, while indirect methods are more sensitive to planets with a short orbital time.
According to Nowak, this opens the door to new studies that can directly map planets with tighter orbits. This is where potentially habitable and “earth-like” (rocky) planets can generally be found. "That means we can now get both the brightness and mass of this exoplanet," he said. "As a rule, the more massive the planet, the brighter it is."
Artist's impression of the ELT using lasers to correct atmospheric disturbances, part of the technique known as adaptive optics. Credit: ESO / L. Calçada / N. Risinger
One of the most anticipated developments for exoplanet studies in the near future is the way next-generation telescopes and instruments are enabling direct imaging studies of tightly bound rocky planets. In this way, astronomers can finally obtain spectra from the atmospheres of the many potentially habitable worlds that have been discovered over the past few decades.
If we judge the composition of the atmospheres of these planets, we can finally say with certainty which of them can support life as we know it.
Further reading: MPI, Astronomy & Astrophysics, A & A.