Help! Our Neptune-sized exoplanets are missing

by | Feb 13, 2023

Astronomers find the “hot Neptune desert” may result from ice-giants migrating away from their parent stars.
Neptune seen from space.

Ever since the discovery of the first planet outside our solar system orbiting a sun-like star, our catalogue of extra-solar planets, or “exoplanets”, has burgeoned, growing to over 5,000 celestial objects in under three decades.

Yet, there remains a frustrating and obvious gap in our observations of exoplanets. Whilst astronomers have observed many exoplanets with sizes around that of Jupiter and greater, small planets the size of Earth, and even smaller worlds similar to Mercury, planets the size of Neptune, which lie between Earth and Jupiter, are missing.

This may be because exoplanet detection methods are fine-tuned to spot exoplanets close to the stars they orbit, and Neptune-sized worlds don’t seem to want to cuddle up to their stellar parents. This problem — the empty box of Neptune-sized planets — has come to be known as the “hot Neptune desert.” 

The hot Neptune desert

A new paper published in the journal Astronomy & Astrophysics presents a potential reason for the hot Neptune desert: a mechanism that causes planets of this size to move away from their star where we can easier spot them with exoplanet detection methods. 

“Most exoplanets known today orbit in less than 30 days around their stars. Among this population of ‘close-in’ exoplanets, we have detected many gas giants with the size of Jupiter, and many of about the size of the Earth, but there is a definite lack of Neptune-size worlds on orbit shorter than about 3 days,” lead author and University of Geneva (UNIGE) researcher Vincent Bourrier said. “We have the means to detect them, so this desert is real. It is a problem because it means Neptune-size worlds form and/or evolve in different ways than other classes of exoplanets.”

Bourrier explained that currently there are two main theories surrounding the cause of the hot Neptune desert. The first suggests exoplanets so close to their stars can lose their atmosphere and Neptune-size worlds might be particularly sensitive to this process, eroding down to their bare cores.

The second, and the one the team focused on, suggests that gas giants the size of Jupiter and Neptune do not form directly close to their stars, but instead migrate from their birthplace farther away. 

“It may be that Neptune-size worlds follow different migration pathways, which makes them arrive at different times or locations close to their star compared to other classes of planets,” said Bourrier.

In an attempt to determine between these theories, Bourrier and the team looked at the “orbital architecture” of fourteen planets located at the edge of this Hot Neptune Desert. They aimed to examine the way their orbits are oriented with respect to the axis of rotation of their star, information that makes it possible to distinguish the processes of soft migration — planets moving in the equatorial plane of their star where they were formed — from the processes of disruptive migration, when the planets migrate and are pushed out of the plane where they were formed.

This revealed that three-quarters of the planets in the sample the team looked at had orbits misaligned with the equator of their parent star, meaning they rotate above the poles of their star. This is a higher proportion of misaligned orbits than is found for planets further from the edge of the Hot Neptune Desert. 

“The findings, that more planets than expected were on misaligned orbits was indeed surprising,” Bourrier explained. “This supports the role of disruptive migration processes in the formation of the desert.”

Orbits provide clues

The team conducted their research by combining two powerful exoplanet observation techniques: the transit method, which observes planets as they cross or transit the face of their star causing a tiny dip in light, and a spectroscopy method which also depends on light from the star to examine the composition of exoplanet atmospheres and other characteristics. 

“We used the high-resolution spectrographs HARPS and HARPS-North located on the 3.6m telescope of the European Southern Observatory and Telescopio Nazionale Galileo to observe the planets during their transit,” Bourrier said. 

He explained that a spectrograph allows astronomers to measure the light a star emits at each wavelength, yielding its spectrum. By comparing this spectrum of the star outside and during the transit, the team could isolate the local spectrum from the regions of the stellar surface as the planet successively occults them during its transit. 

Because a star rotates, their local light spectra are shifted in wavelength according to their local velocity, a phenomenon exactly like the Doppler effect, but with sound. An example of the Doppler effect is the changes in the sound of an ambulance as it approaches and then moves away from you, becoming distorted by the ambulance’s motion as the sound waves stack against one another.

The same thing happens with light — as an object moves away from Earth like the soundwaves of a receding ambulance siren, the wavelength of light is stretched. This causes it to become redder in color, as red light has longer wavelengths, hence why this is called redshift. Conversely, when an object moves towards Earth, like the approaching siren, wavelengths of light are squashed moving towards the short wavelength or blue end of the visible light spectrum, a phenomena called blue shift.

By analyzing these shifts in spectra of a star on a local level, the team was able to determine which regions of the stellar surface a planet is transiting, thus inferring the orientation of its orbit.

The local approach taken by the scientists is far more direct than looking at the global changes in spectra caused by the transit of a planet and gives a more precise picture of the distortion it causes and thus the planet’s orbit.

The team will now apply this technique to the smallest planets at the edge of the Hot Neptune Desert to attempt to fully solve this exoplanet enigma and determine if migration is responsible for it.

“We are now looking at all existing measurements of orbital orientations for planets close to their star, to see if we can unveil trends with the properties of the planet and star properties that might help us better understand the processes that shaped their orbit,” said Bourrier.

“One of the questions that remain unanswered is the interplay between migration and atmospheric escape, which is the main focus of the project my team is working on, thanks to funding granted by the European Commission (an ERC starting grant),” he concluded.

Reference: V. Bourrier., O. Attia., et al., DREAM, I. Orbital architecture orrery, Astronomy & Astrophysics, (2023). DOI: 10.1051/0004-6361/202245004

Feature image credit: NASA