By Jérôme Bouvier & Rajeev Manick
Our ability to detect planets outside our Solar System is becoming more refined. Thus,
astrophysicists can now detect exoplanets still nestled within the dust cloud that witnessed their birth, right after their formation. Such is the case with this discovery: a very young planet orbiting very close to its star.
Around a star only 2 million years old, astrophysicists were able to detect the presence of a newly formed planet orbiting very close to its sun.
NASA/JPL-Caltech, adapted by Kritish Kariman
The search for
exoplanets, planets that orbit stars other than our Sun, is one of the key challenges in modern astrophysics. A new milestone has been reached with the
detection of a nascent planet in close orbit around a young star. It is currently the youngest and most compact system detected at the moment of its formation, opening a new window into the origin of exoplanetary systems.
In 1995, Michel Mayor and Didier Queloz detected the first exoplanet orbiting a Sun-like star at the Haute-Provence Observatory: 51 Pegasi b. The Nobel Prize awarded to the two Swiss astronomers in 2019 highlights the importance of this discovery, which offers new insights into our origins and the possibility of life elsewhere.
Since then, an entire branch of astrophysics research has been devoted to searching for exoworlds. To date,
nearly 7,000 exoplanets have been identified around stars in our region of the Galaxy. By extrapolation, it is estimated that almost every star in the Galaxy has one or more planets.
Tracking the birth of planets
Over the last decade, NASA's
Kepler space telescope surpassed expectations: it alone has detected several thousand planets, revealing the architecture of exoplanetary systems. Compared to our Solar System — composed of four inner rocky planets and four outer gas giants — most extrasolar systems appear much more compact.
Model of NASA's Kepler space telescope. NASA/Wikimedia
Exoplanetary systems generally host planets of the super-Earth type (between 1.2 and 3.5 times the size of Earth) and mini-Neptunes (up to 7 or 8 times the size of Earth), orbiting very close to their star, often closer than Mercury's orbit around the Sun (about 37 million miles [60 million kilometers]). Such is the case with the Trappist-1 exoplanets. To better understand the origin of these systems, it is necessary to attempt to detect them at the very moment of their birth.
The goal of the European project
SPIDI, which we are leading at the
Institute of Planetology and Astrophysics in Grenoble (IPAG), is precisely to detect forming exoplanets around young stars. In particular, we are searching for exoplanets in close orbit, precursors to the compact systems revealed by Kepler around already mature stars.
However, several challenges still need to be overcome. Due to their distance, several hundred light-years away, their tight orbit around the star, and their low luminosity, we are not yet able to directly perceive the light coming from these compact systems.
Cutting-edge instruments to detect the presence of a planet
It is therefore necessary to rely on indirect methods, such as looking for disturbances in the movement or radiation of the star induced by the planets. The main difficulty then lies in the high activity of young stars, where eruptive phenomena occur that are a thousand times more violent than on the surface of our Sun. Trying to detect a planetary signal lost in the "noise" of the star is therefore like attempting to listen to a symphony near a jackhammer.
Only by using the most advanced instruments available today were we able to detect a signal betraying the presence of a new exoplanet. We notably made use of the
Canada-France-Hawaii telescope, located 4,200 meters above sea level in the middle of the Pacific, combined with data from the Kepler satellite and the use of a
telescope network distributed all around the Earth, the
Las Cumbres Observatory.
The Canada-France-Hawaii Observatory at the summit of Mauna Kea, Hawaii.
Generic1139/Wikimedia, CC BY-SA
All of this data allowed us to determine that this planet orbits in less than a month around a
young star named CI Tau. The detected signal takes the form of periodic variations in the system's brightness and velocity, repeating every 25.2 days. The star rotates on itself in only 9 days, which was not sufficient to explain the observations.
A celestial body still nestled in the cocoon that gave it life
Located in the constellation of Taurus, this star, only 2 million years old — the equivalent of a few days on a stellar scale — is still surrounded by its protoplanetary disk, a disk of gas and dust around the star in which planets are forming. The structure of this disk is segmented, suggesting the possible presence of other planets. However, so far, we have only detected one. Since each technique has its own detection biases, we often need to use several to complete the description of the system.
This is a hot proto-Jupiter, with an estimated mass of 3.6 times that of Jupiter, orbiting its host star in a highly eccentric orbit. It is during this period that the architecture of these systems is determined, a result of the interaction between the star and its disk. This first discovery of an exoplanet orbiting less than 15 million miles [25 million kilometers] from its star, interacting with its disk, complements the
direct detection of nascent planets orbiting their host star at a distance of several billion kilometers (billions of miles).
Still buried in the disk that gave birth to it, this planet demonstrates the possibility of studying the formation phases of compact exoplanetary systems, which seem to populate the Galaxy.
In fine, studying this type of system will help clarify the initial conditions that govern the formation of exoworlds, through rich and complex processes, some of which may lead to the emergence of life.