My research focusses on the study of brown dwarfs and extra-solar giant planets with direct imaging, a method that allows us to see these objects directly (instead of observing their effects on their host stars). I am especially interested in looking at known planetary systems and isolated brown dwarfs (not orbiting stars) to achieve a more complete overview of their characteristics and demographics. Studying these objects as a broad family can tell us a lot about their fundamental properties, how they formed and evolved, or what to expect from their atmospheres.

Check out some of my recent work below:

  • Project 1:  Giant Planets around Brown Dwarfs
  • Project 2:  Brown Dwarf Multiplicity Statistics
  • Project 3:  Precise Distances of the Coldest Brown Dwarfs
  • Project 4:  Exoplanets in Binary Star Systems
  • Project 5:  Finding Directly-Imaged Companions with COPAINS
  • Project 6:  Occurrences of Wide-Orbit Gas Giant Planets

Giant Planets around Brown Dwarfs

Using the Hubble Space Telescope (HST), I discovered CFHTWIR-Oph 98 (Oph 98 for short), a curious starless binary system, consisting of two very low-mass objects located 450 light years away from Earth. The more massive component, Oph 98 A, is a young brown dwarf with a mass of 15 times that of Jupiter. It was found to host a giant planetary companion, Oph 98 B, only 8 times heavier than Jupiter, and separated by 200 times the Earth-Sun distance. This provides a rare example of two objects similar in many aspects to extra-solar giant planets, but orbiting around each other with no parent star.

Fontanive Oph98 HST binary
The young Oph 98 planetary-mass binary, on a wide 200-au separation, observed with different telescopes.
Fontanive et al. 2020.

Although Oph 98 A and B have masses, temperatures and atmospheres comparable to those of gas giant planets orbiting around young stars, they most likely formed like stars. The discovery of Oph 98 not only provides two new precious examples free-floating worlds analogous to giant exoplanets, but also proves that such extremely low-mass objects can have giant planetary companions, and that the processes that create binary stars operate on scaled-down versions all the way down to these planetary masses.

Read the Press Release here!

Fontanive Oph98 Thibaut Roger
Artist's impression of the two brown dwarfs, with Oph 98 B in the foreground in purple, Oph 98 A in the background in red. Oph 98 A, the more massive component, is more luminous and hotter. The two objects are surrounded by the molecular cloud in which they were formed.
© Image: University of Bern, Thibaut Roger.

Brown Dwarf Multiplicity Statistics

Several theories exist to explain the existence of brown dwarfs floating freely in space without a host star, but it is still unclear whether some of them are born like stars or rather like planets. Population studies provide very valuable insight into formation histories, making them critical for differentiating between proposed formation and evolution scenarios.

About half of Sun-like stars are in binary systems, with two stars orbiting each other. Smaller stars are less commonly in multiple systems, are in tighter binaries and show a clear tendency towards equal-mass systems. Using the Hubble Space Telescope images, I demonstrated that the trends seen in the binary statistics of stars continue across the substellar regime, persisting down to the very lowest-mass brown dwarfs. This observed continuity between the binary frequencies and population distributions of star and brown dwarf binaries suggests a common formation mechanism for the stellar and substellar regimes, providing crucial information to test formation models.

Fontanive brown dwarf binary fraction
Trend of decreasing binary fraction with stellar mass extending to the bottom of the brown dwarf regime.
Fontanive et al. 2018.

Precise Distances of the Coldest Brown Dwarfs

The distance to an astronomical object is a fundamental property that is absolutely necessary to know in order to correctly interpret observations. Y dwarfs are the faintest and coldest type of brown dwarfs, and with similar masses and temperatures to giant exoplanets, provide ideal proxies to study planetary atmospheres. However, distances are very challenging to measure for such faint targets, which is particularly problematic when trying to understand their complex characteristics.

Astrometry, the science of measuring positions and motions of celestial bodies, allows to estimate parallax angles, which are a direct measure of an object's distance from us. I devised a new method to improve the astrometric precision of Hubble observations in order to derive the most precise parallaxes for very faint Y dwarfs, by combining HST images taken over several years with the Gaia mission. These highly-precise distances will allow us to robustly calibrate observations of Y dwarfs and study in great details the atmospheres of these pivotal giant planet analogues.

Fontanive Bedin astrometry brown dwarfs HST
Motion of a brown dwarf in the sky in HST images taken over 6 years.
Bedin & Fontanive 2020.

Exoplanets in Binary Star Systems

Stellar multiplicity is believed to influence planetary formation and migration, although the precise nature and extent of this role remain ambiguous. I conducted several surveys to test the impact of stellar binaries on the formation and evolution of various types of exoplanets, which provide important information to better understand planet formation and improve theoretical simulations.

In a first paper, I showed that stars hosting massive planets and brown dwarfs on very short separations are almost all found in multiple-star systems, demonstrating that binarity plays a crucial role in the existence of high-mass and close-in exoplanets. In another study, I compiled the largest list to date of exoplanets in stellar binaries. With this catalogue, I found that the properties of small and wide-orbit planets are not affected by the presence of companion stars, unlike high-mass close-in planets. I also show that very wide binary systems do not impact the planetary systems.

Fontanive exoplanets binary stars
Compilation of exoplanets in stellar binaries.
Fontanive & Bardalez Gagliuffi 2021.

Finding Directly-Imaged Companions with COPAINS

The wobble of a star in its orbit induced by the gravitational pull of a companion is one of the most efficient ways to detect that companion. When the movement is in the plane of the sky, it can be visible as a change over time in the star’s motion across the sky. A non-linear apparent trajectory may thus serve as evidence for the existence of an unseen companion.

Fontanive COPAINS astrometry imaging

To exploit this, we developped COPAINS, a new tool to identify previously undiscovered companions based on changes in stellar motions. It allows us to predict the possible masses and separations of hidden companions compatible with observed proper motion offsets. Comparing the predictions of the code to the expected sensitivity of various imaging instruments, this in turn enables us to robustly select the most promising targets for direct imaging campaigns searching for low-mass companions, with the hope to increase the current census of wide brown dwarfs and giant planet companions to stars, which remain very rare in current surveys.

Fontanive Bonavita HIP 21152 brown dwarf SPHERE
Detection of a brown dwarf companion (in the red circle) around the star HIP 21152 with SPHERE.
Bonavita, Fontanive et al. 2022.

In a pilot survey, we selected with COPAINS 25 stars with trends from Gaia data compatible with a low-mass secondary detectable in direct imaging observations. We observed them with the SPHERE instrument at the Very Large Telescope, and detected 10 new companions: 5 low-mass stars, 1 white dwarf, and 4 brown dwarfs. Around 40 directly-imaged brown dwarf companions had been found after almost three decades of searches, and our results strongly increase this number, by 10% in one go, with a significant boost in brown dwarf detection rates compared to typical 'blind' imaging surveys.

Read the Press Release here!

Fontanive COPAINS Bonavita brown dwarfs
Artist’s impression of the companions discovered in the COPAINS survey.
© Image: University of Bern.

Occurrences of Wide-Orbit Gas Giant Planets

The direct imaging method is the only way to detect a planet based on its own emitted light, rather than through the effect of the planet on its host star, but is only sensitive to very massive planets orbiting from very large distances. Constraining the demographics and architectures of giant exoplanets and brown dwarfs on wide orbits allows to test fingerprints of formation mechanisms, and provides rich sources of empirical constraints for planet formation and migration simulations.

SHINE (SpHere INfrared survey for Exoplanets) is a large dedicated direct imaging survey using the high-contrast imager SPHERE at the Very Large Telescope (VLT), to perform a census of young giant exoplanets in the Solar neighbourhood. I led the statistical analysis of the first 150 stars observed as part of the SHINE survey. Comparing observations to theoretical models, we found that more massive stars produce more giant planets, and low-mass stars have more brown dwarf companions.

Fontanive Vigan exoplanets imaging SPHERE SHINE
Planets and brown dwarfs detected in the SHINE direct imaging survey.
Vigan, Fontanive et al. 2021.