Review written by Vyshnavi Vennelakanti (Mechanical and Aerospace Engineering, Postdoc)
Have you ever wondered how planets are formed? Current theories of planet formation suggest that they likely form at the same time as their host stars from the same initial reservoir of gas and dust. All newly formed stars have a rotating disk around them called the protoplanetary disk. This disk contains lots of particles and gas molecules. There are two main theories that aim to explain planet formation from the protoplanetary disks of stars: the core-accretion theory and the gravitational instability theory.
The core accretion theory states that these dust particles collide with each other to coalesce into larger objects which are often held together by the presence of gas molecules in the disk. This process of particle accumulation continues over millions of years, resulting in planet formation.
The gravitational instability theory, on the other hand, states that pockets of gas in the protoplanetary disk become extremely dense, resulting in a decrease in temperature followed by a rapid contraction which then forms a planet. Planets formed through gravitational instability are usually gas giants which are extremely massive, thus experiencing a larger gravitational force of attraction from the star. As a result, scientists believe that planet formation through gravitational instability can only take place much farther away from the star so that the star’s gravity does not rip the planet apart.
While we have two main theories to explain planet formation, how can we directly detect planet formation? We can observe planets in our solar system such as Saturn and its rings through a telescope (a form of direct imaging of a planet). However, planets outside our solar system — i.e., exoplanets — can be indirectly detected with a method called the radial velocity method. This method relies on the fact that a star and its planet are both gravitationally attracted towards each other, resulting in both celestial bodies orbiting their center of mass. Since the star is much more massive than its planet, the center of mass for this two-body system is located within the star. Thus, the planet orbits the star while the star merely wobbles as a result of gravitational attraction. Scientists leverage the radial velocity method to observe the star’s wobble which further provides more information about the planet(s) orbiting the star.
The radial velocity method can be used to detect multiple planets around a star. Each planet has a specific orbital period, i.e., how long it takes for the planet to go around the star, which for the Earth is approximately 365 days to go around our Sun. When there are multiple planets, each planet's gravitational pull will contribute to the overall radial velocity of the star. Analysis of the radial velocity measurements can help differentiate between the signals of multiple exoplanets of a star. While the combination of signals from multiple planets creates a complex pattern, the unique orbital period of each planet makes the pattern predictable.
A survey designed to search for planets around very low-mass stars led to the observations of LHS 3154, a dwarf star and its planet, LHS 3154b. A team of researchers in the department of Astrophysical Sciences at Princeton University, led by Dr. Stefansson, observed LHS 3154 which is located approximately 51.4 lightyears away from the Sun, outside our solar system. LHS 3154 is an extremely low mass star, with a mass 9 times less than our Sun and is therefore extremely cold as far as stars are considered. The Habitable-zone Planet Finder spectrograph, which was specifically designed to detect and characterize planets around low-mass stars, was used to observe LHS 3154. After obtaining 137 spectra which present information about the frequencies of light emitted by LHS 3154 over a span of two years, the researchers were able to measure the star’s radial velocity.
The plot of radial velocity shows periodic changes in radial velocity as a function of time, likely a result of the star’s wobble due to its planet’s gravity (Figure 1). If a planet is orbiting its star, we can think of the planet at a position named X, and the star at a position named Y (Figure 2). The star has a radial velocity of R on Day 1 (Figure 2). As the planet orbits the star, the star experiences the planet’s gravity and wobbles. As a result of this, the star’s radial velocity changes with time as the planet moves in its orbit around the star. When the planet completes an orbit and is back to position X and the star to position Y, the star’s radial velocity will once again be back to R (Figure 2). The time taken for the velocity to come back to R gives the orbital period of the planet. For LHS 3154, the planet’s orbital period was found to be 3.7 days from the radial velocity plot as a function of time. Additionally, Dr. Stefansson and coauthors were the first to compute the mass, radius, and temperature of the star along with other properties based on their measurements from the collected spectra and inferences from prior studies. Several parameters for the planet in addition to the star’s parameters are evaluated in this work, revealing that the planet LHS 3154b is the most massive planet so far detected to orbit such a low mass star. The discovery of this surprisingly large planet around such a small star immediately raises a question: how might it have formed?
Figure 1. A representative plot showing the radial velocity of a star as a function of time.
Figure 2. A figure showing a star and its planet on two days. On Day 1, the position of the planet is at X and that of the star at Y, and the star has a radical velocity of R. On Day n, the position of the planet is at X’ and that of the star at Y’, and the star has a radial velocity of R’.
Immediately, the team set out to explore LHS 3154 b’s possible origins: did it form via core accretion or via gravitational instability? The team performed extensive simulations of the core-accretion model assuming nominal protoplanetary disk dust masses, which suggested that the planets formed by core accretion around stars as low mass as LHS 3154 are much lighter than LHS 3154b by almost an order of magnitude. Therefore, formation of LHS 3154b via core accretion appears to be a challenge.
The team then considered whether the gravitational instability model could explain the formation of LHS 3154b. The orbit of this planet was measured to be 3.7 days. However, planets formed through gravitational instability are usually on much wider orbits than this, and simulations of gravitational instability around a star with mass comparable to LHS 3154 reveal that planets formed around this star tend to be much more massive than LHS 3154b. While this model cannot be completely ruled out to explain the formation of LHS 3154b, if this planet was indeed formed as a result of gravitational instability followed by migration inwards (towards the star), it would have required much greater disk masses than those considered in core-accretion models.
The results of simulations are known to be sensitive towards the assumptions about the total disk mass and disk surface density. Thus, the authors of this work performed simulations by varying these two parameters to understand the properties of planets predicted in these simulations, i.e., if a certain combination of these parameters predicts planets with properties similar to that of LHS 3154b. They concluded that substantially more massive protoplanetary disks in combination with more compact disks can result in the formation of planets with properties similar to LHS 3154b, i.e., massive close-orbiting planets.
Given the challenges for both theories of planet formation to describe the origins of LHS 3154b, more analysis is warranted. Both planet formation models in this case require much higher disk masses than typically observed for very low-mass stars — by at least 1-2 orders of magnitude. The authors of this paper thereby proposed, but have not yet tested, three possible explanations for how the planet LHS 3154b might have formed: 1) the disk dust masses might have been underestimated during observations — this is a possibility if a large fraction of the dust in the disk grows to centimeter-sized grains which would not be detected by the millimeter observations used to estimate protoplanetary dust masses; 2) disks accrete additional material from the surrounding parent molecular cloud; 3) the planetary cores form much sooner, e.g., within a million years, when the disks are more massive than at later times. All three proposed scenarios rely on heavier disk masses in which case either of the main planet formation models would be able to explain the origins of LHS 3154b. The larger disk masses along with disk density can explain the formation of LHS 3154b by core-accretion model as also suggested by the core accretion model simulations. Gravitational instability can also explain the formation of this planet provided higher disk masses in which case, the planet would have formed much farther away from the star and then migrated inwards to a closer orbit. For either formation scenario, the discovery of LHS 3154b demonstrates that protoplanetary disks around the lowest mass stars need to be substantially more massive to explain the existence of such massive planets around the lowest mass stars.
Dr. Guðmundur Stefansson continues to survey for other planets in his own lab at the Anton Pannekoek Institute for Astronomy of the University of Amsterdam. “The discovery of LHS 3154b is an extremely interesting find,” he shared with us, “as it reshapes our ideas of how planet formation occurs around the lowest mass stars.” Dr. Stafansson said the research team was “particularly surprised” about the discovery of LHS 3154b, “as we were on the lookout to find rocky planets possibly in the so-called habitable-zone (the region around a star where liquid water can be sustained on the planet); but on that quest, we found this one…As we continue the survey for other planets, we are keeping a lookout for oddball discoveries such as this one.”
The original article discussed here was published in Science on Nov 30, 2023. Please follow this link to view the full version.