Stars and planets: Born of fire and ice

A deep look into the universe

An image taken by the Euclid space telescope shows the reflection nebula Messier 78, a vibrant nursery of star formation enveloped in a shroud of interstellar dust. Messier 78 lies 1,300 light-years away in the constellation of Orion.

Some are infernally hot – hotter than the most common stars in our Galaxy. Others move so incredibly fast that their year lasts only a little over four hours. Still others get rained on – not by water, but by glass. And then there are the ones that, at least at first glance, are virtually indistinguishable from our Earth.

Exoplanets, i.e. planets that circle distant stars, are always good for a surprise. Astronomers have so far discovered nearly 5,900 of these strange worlds – in almost every conceivable variation, despite the fact that planetary systems are all born of the same fundamental building blocks.

“The most important ingredients are simply gas and dust,” says LMU Professor Barbara Ercolano. If you then add icy cold followed by a huge helping of heat, the planets take shape more or less on their own. That said, what exactly happens in the cradles of exoplanets, what conditions prevail there and how all this affects the vast planetary diversity is still a major conundrum. Ercolano, futurespokesperson for the ORIGINS Excellence Cluster, is therefore working with her group to shed light on the murky mechanisms of planetary genesis – and ultimately to understand how common and how simple it is for a solar system similar to our own and with a livable second earth to take shape.

Searching for exoplanets

Astrophysicist Barbara Ercolano at the ESO Supernova Planetarium in Garching near Munich

Planets with peculiar properties

Exoplanets are a decidedly recent field of research. And we are not talking about astronomical time scales: A mere 30 years or so ago, astronomers only knew what, at the time, were the nine planets of our own solar system. It was not until 1995 that the first exoplanet was discovered in the vicinity of another star similar to our sun.

Since then, there has been no stopping the growth of this discipline. Telescopes and space probes have helped researchers identify many thousands of exoplanets. However, the often strange properties of these heavenly bodies raise more questions than answers. “For me, precisely this diversity was one of the reasons why I wanted to take a closer look at the birthplaces of planets,” Ercolano says.

»For me, precisely this diversity was one of the reasons why I wanted to take a closer look at the birthplaces of planets.«

Barbara Ercolano

For an exoplanet to come into existence, you first need a star. And in this regard, astrophysical models are already far more advanced and elaborate: A dense interstellar cloud of dust and, above all, gas is required. Hydrogen molecules in particular probably play a pivotal role. They have to be cold. Extremely cold, in fact, at around minus 250 degrees Celsius. Only then can mutual attraction occur between the gas and dust particles, which densify and ultimately collapse under their own weight to form what is called a protostar.

This collapse in turn causes the temperature inside the clump of gas and dust to rise sharply. If it reaches several million degrees Celsius, the hydrogen fuses into helium, which is heavier, and this process releases a lot of energy. A star is thus born and begins to shine.

However, not all the dust and gas in the interstellar cloud becomes part of the star. Some debris is left behind and accumulates in a region rotating around the new-born sun. Astronomers speak of a protoplanetary disk – the cradle of exoplanets.

How planets are born

Illustration of a model showing how gas giants such as Jupiter, Saturn or Uranus could also form quickly in the solar system from the dust of a protoplanetary disk and then drive dust into areas outside their orbit.

No dust? No planets!

It is possible that similar processes to those during the birth of the star itself take place in this disk: Dust and gas are mutually attracted and coagulate. The disk becomes unstable, and another cluster or lump emerges – the only difference being that the mass of this ball of gas and dust is not sufficient to fan the flames of nuclear fusion. What is left over is a planet.

Today, most researchers nevertheless tend toward a different explanation: They postulate that the tiny dust particles in the disk collide, stick to each other and grow in size – from micrometers to begin with to spheres with a diameter of several kilometers. These lumps can attract further material until fully grown exoplanets finally emerge. “According to this theory,” Ercolano explains, “there are no planets if you don’t have dust.”

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The fly in the ointment? The most common current models reckon that the whole process, known to scientists as core accretion, takes up to 100 million years. However, practically all known protoplanetary disks dissolve again at the latest after ten million years.

Birth of exoplanets

It is to this inconsistency that Ercolano has applied herself for the past 15 years. One possible solution could be supplied by radiation – especially in the x-ray and extreme ultraviolet range – emitted from the young star. If this high-energy radiation hits the outer reaches of the disk, it can effectively evaporate the material it finds there. Photoevaporation is the proper name of this process. The rotation of the disk would then sling the evaporated gases out into space.

But hang on a moment: Wouldn’t that prevent the birth of exoplanets? Not necessarily, Ercolano contends. If the radiation removes the gas, comparatively more dust is left behind in the disk. And this could, in particular, lead to the emergence of more rocky, earthlike planets. “Photoevaporation undoubtedly stops the ongoing formation of the planets at some point. In the meantime, though, it could actually reinforce the process,” the astrophysicist says. “Either way, it certainly has a considerable influence.”

Ercolano is a theoretician. Models and numeric simulations are her world: a world of magnetic fields, radiative transfer and hydrodynamic processes. Which is not to say that the native of Naples divorces her work from real observations. On the contrary: Ercolano’s group spends a lot of time generating virtual images on the basis of her models. She calculates which pictures existing scientific instruments would deliver on the basis of the given theories and then compares these with real data. “Synthetic observations” is how she describes the method.

Very Large Telescope (VLT) at the European Southern Observatory in Chile

Glimpse into the universe

Gigantic telescopes such as the VLT at the European Southern Observatory in Chile have produced this kind of spectroscopic image of protoplanetary disks.

The fingerprint of planetary genesis

Her focus is on what are referred to as spectroscopic images: When a star’s radiation encounters gases such as oxygen in the protoplanetary disk, the molecules are excited and themselves emit light with characteristic wavelengths – a kind of fingerprint. If these gases are then ejected from the disk in the direction of observers here on Earth, this modulates the lines in the fingerprint toward slightly shorter wavelengths.

Gigantic telescopes such as the VLT at the European Southern Observatory in Chile have produced this kind of spectroscopic image of protoplanetary disks. Ercolano has placed her own simulations, her synthetic observations, alongside these images: The two were almost entirely congruent – powerful testimony to the validity of the theoretical models. “If the lines agree, the temperature is right too, and so is the incidence of gases in our simulations,” she notes.

That said, it is still not clear whether x-rays and extreme ultraviolet radiation are primarily responsible for the fate of the disk and, hence, for the birth of exoplanets. Other research groups have concentrated on lower-energy ultraviolet light but have also been able to achieve close conformity with the observational data. “It remains to be seen which model, if any, matches reality,” Ercolano concedes. Right now, her research group is therefore working to feed additional types of radiation into its simulations.

LMU involved in space mission

Munich scientist in the Twinkle project have exclusive influence on the mission. “That is fantastic,” Ercolano exclaims. “It is something we can only dream of with telescopes such as the James Webb.”

»Twinkle is the first spectroscopic space observatory designed specifically to study the atmospheres of exoplanets.«

Barbara Ercolano

In recent years, another project – Twinkle – has been added. Twinkle is a small space telescope whose purpose is to systematically scour exoplanets for conditions that may be conducive to life. The mission, which could begin in 2027, is the brainchild not of government space agencies, but of British company Blue Skies Space and a whole series of universities. Munich’s ORIGINS Excellence Cluster is one of the founder members.

“Twinkle is the first spectroscopic space observatory designed specifically to study the atmospheres of exoplanets,” Ercolano says. A mirror diameter of only 45 centimeters makes Twinkle relatively small, though. Both its resolution and its sensitivity are fairly low, especially compared to giants such as the James Webb Space Telescope (JWST), which was launched at the end of 2021. Armed with a 6.5-meter mirror, the JWST too will help investigate the atmospheres of exoplanets. But it is also charged with many other astronomic and cosmological tasks, so researchers have to vie for scarce observation time. Demand is huge, and success is uncertain.

On the other hand, the few organizations involved in the Twinkle project have exclusive influence on the mission, on observation times and on the planets to be observed. “That is fantastic,” Ercolano exclaims. “It is something we can only dream of with telescopes such as the James Webb.”

Searching for signs – and signatures – of life

In particular, Twinkle will hunt for organic molecules such as methane and ammonia, alongside hydrocarbons, in distant atmospheres. Such molecules are regarded as possible precursors of the emergence of life as we know it. And, rather like the gases in the protoplanetary disk, they reveal themselves by leaving a characteristic fingerprint in starlight. Ercolano and her team have already begun using statistical methods and artificial intelligence to craft models of atmospheres based on different compositions and the resultant observation data. These models will then be juxtaposed with real Twinkle images to unpack the details of these atmospheres.

In all probability, Twinkle will not yet be able to provide direct evidence of biosignatures – spectroscopic fingerprints that point to life in an atmosphere and rank as the Holy Grail of the search for exoplanets. That said, the mission could indeed deliver initial clues and gather valuable experience that might clear the path to future discoveries.

Prof. Dr. Barbara Ercolano is an astrophysicist at LMU and future spokesperson for the ORIGINS Excellence Cluster. She conducts research into protoplanetary disks and the genesis of planets.