Adriano Fontana, Research Director, INAF – Osservatorio Astronomico di Roma, Istituto Nazionale di Astrofisica

Abstract

What are stars and planets? How did the Universe begin? Is there a connection between our Earth and the sky? These and other similar fundamental questions have tormented the curiosity of mankind since the birth of our species. These questions have started to be addressed with the development of modern astronomy, which began in 1610 when Galileo Galilei first put his eye to the telescope. Despite four centuries of discoveries, however, many of these fundamental questions remain unaddressed, and are still the subject of active research. I present here a succinct and simplified view of some of the major accomplishments of modern astronomy, as well as the fundamental questions that are still open, concerning the existence of life on other planets and the nature, birth and fate of the Universe as a whole.

I conclude mentioning how the recent development of mega-constellations of low orbit satellites threaten future observations of the sky from the Earth.

Introduction

Along with medicine and botany, astronomy is arguably one of the oldest human sciences. It stems from the inevitable curiosity about the nature of the objects that we see in the sky and of their complex motion: the daily setting of the Sun and the Moon, the apparent erratic movements of the planets, the yearly motion of the fixed stars. For thousands of years, mankind – not overshined by artificial lights – has stared at the sky and argued about its role and origin. All human cultures in every continent have developed their explanations about the birth and nature of the Sun, the Moon, and planets. In particular, the luminous stripe that crosses the summer sky (see Figure 1) has attracted human attention and given rise to a number of explanations about its birth, resulting in a variety of different names – “Milky Way” in the Western Culture, “River Ganga in the Sky” in Sanskrit, “Silver River” in Chinese, “River of Heaven” in Japanese, “Straw Way” in a number of Asian cultures, “Fish jumping in shadows” in some Hawaiian cultures, “The Heavens River” in some Berber languages, just to mention a few.

Ancient astronomers mastered the motion of objects in the sky and used this knowledge to predict the change of seasons and take informed decisions about harvesting, migrations and other human activities.

Since its origin, astronomy has therefore been linked to the human quest of the origin and ultimate nature of the world, has influenced the perception of mankind’s position in the world and influenced its daily life.

Modern astronomers are the heirs of this tradition and are moved by the same ultimate goals. While we nowadays use powerful instruments to explore beyond what is visible using only the naked eye and utilize sophisticated mathematical analyses to decipher the rules that govern the laws of the Universe, they are driven by the same curiosity and fascination that moved every human being. The sense of wonder in discovering the secrets of the Universe and the desire to understand how the Universe and the world around us were borne are the same that motivated ancient astronomers and that attract all of us.

Galileo and Hubble: exploring the Universe

There is no better evidence of this motivation than the words used by the giants of modern astronomy to describe their discoveries.

Galileo Galilei founded modern astronomy by using for the first time an instrument – a small telescope that was used previously only for navigation purposes – to look at the sky. In his masterpiece Sidereus Nuncius, published in 1610, Galileo reports his discoveries about the surface of the Moon, the existence of satellites around Jupiter and the nature of the Milky Way. It is worth looking at his hand-drafted designs (Figure 2) and reading how he describes his discoveries:

In third place, I have observed the essence or substance of the Milky Way circle. By the aid of the telescope anyone may behold this in a manner which so distinctly appeals to the senses that all the disputes which have

tormented philosophers through so many ages are exploded at once by the unquestionable evidence of our eyes. […] the Galaxy is nothing else but a mass of innumerable stars planted together in clusters. Upon whatever part of it you direct the telescope straightway a vast crowd of stars present itself to view; many of them are tolerably large and extremely bright, but the small number of small ones is quite beyond determination.

In the first sentence, Galileo proudly announces the superiority of his new observations based on the power of new instruments and the undisputable evidence upon which scientific method is based. In the second sentence, the sense of wonder for his discovery clearly emerges. We can only imagine the astonishment that Galileo must have felt when he pointed his telescope toward the fuzzy luminosity of the Milky Way and discovered that it was made of “a vast crowd of stars”: literally a new world emerged in front of his eye.

With his discovery, Galileo proved for the first time that the Milky Way is a cloud of “innumerable” stars, and that the Sun in only one star in a vast island of stars. Over these 400 years from Galileo’s discovery, astronomers have proved that the Milky Way is made of approximately 100 billion stars (most smaller but some larger than the Sun), that it has the shape of a rotating spiral with a central spherical bulge, and that the Sun is located in an undistinguished position relatively far away from the galaxy center.

Three hundred years after Galileo, another astronomer revolutionized our understanding of the Universe: in 1921, Edwin Hubble discovered that our galaxy is only one among billions of other galaxies: the Milky Way is not the only “island of stars” in the Universe but – again – innumerable others galaxies exist. A few years later he also discovered that all external galaxies run away from our Milky Way, and those farther away are also faster – this has been the first evidence that the Universe is constantly expanding. Again, his words are worth reading [2]:

Telescopes have continued to develop until today we are exploring those outer regions. They are inhabited by stellar systems comparable with our own system of the Milky Way. Those other systems are the extragalactic nebulae (galaxies). We find them scattered thinly through space out as far as telescopes can reach. […] They are gigantic beacons, permitting us to survey and study a sample of the universe. Eventually, if the sample is fair, we will be able to infer the nature of the universe as a whole from the observed characteristics of the sample available for inspection. This possibility is the ultimate goal of the space exploration.

The first sentence is incredibly akin to Galileo’s first sentence – it explains that discoveries are made possible by new instruments and conveys the sense of wonder for the discovery of galaxies “scattered thinly through space out as far as telescopes can reach”, exactly like Galileo’s discovery of stars in the Milky Way.

The last sentence clarifies that the ultimate goal of this effort is “to infer the nature of the universe”: the same desire and curiosity that arose in early astronomers is motivating their modern descendants.

In the last century, after Hubble’s discoveries, astronomers have continued to explore the Universe and tried to pin down the fundamental laws that govern it. In this process, we have expanded our view in an unprecedented way and discovered new fundamental properties of the Universe. As always in science, these discoveries have opened new questions and unveiled new mysteries that await explanation. In the following I will give a few examples of what we have discovered – and what not (yet).

Planets around other stars.

Until less than 30 years ago, the only known planets were the eight orbiting the Sun, five of which are visible to the naked eye. In 1995, the first discovery of a planet orbiting another star was obtained [3]. This discovery was awarded a Nobel Prize in 2019, as one of the major breakthroughs in modern science. Since then, more than 5000 planets have been discovered around other stars among those close to the Sun, with a variety of techniques and instruments. These objects have been collectively called exoplanets to emphasize their location outside our Solar System. For most of these objects we have solid measurements of their fundamental characteristics like mass, radius or distance from the parent star. From these, we can infer their composition and their surface temperature. For a few of them we also have direct images or even measurements of the chemical composition of their atmosphere (see Figure 3 for an example). These observations have transformed our view regarding the existence and nature of planets around other stars.

It has now become clear that the formation of planet(s) is a very common process. To the point that we now believe that most – if not all – the stars in the Universe have planets orbiting around them. It is also very clear that planets come in a wide variety of sizes, shapes, and chemical compositions: we have hardly observed two planets that have similar characteristics. Some planets are larger than Jupiter and much hotter, others are smaller and closer to their star than Mercury. Some are rocky deserts, other are covered with water oceans, other are fluffy giants mostly made of gas. Some are covered by a thick layer of clouds, other have a bare surface with nearly no atmosphere.

We have also learned that planets can migrate in their system, i.e. they can get closer or farther from their parent stars while their solar system is forming. And we have also learned that planet formation is an event that occurs relatively quickly along with the formation of the star – likely within 50 million years, quite a brief time in comparison with the life of a star, which may easily be a billion years.

Despite these discoveries, there are many fundamental aspects that we have not yet understood.

First, we are not yet able to observe planets like the Earth orbiting other stars. In practice, if we were observing stars with our instrumentation in our solar system from another star, we would be unable to detect any planet in our solar system. The planets that we have observed so far are either much larger and/or closer to their sun than those in our solar system.

Due to the lack of information about earth-like planets, we have not (yet) understood how planets form around stars. Some theories predict that they form directly from the gas that also formed the central star, other from “pebbles” – namely big rocks that hit one other and eventually merge into a larger planet. Or, possibly, by some combination of these two avenues. Because of these two uncertainties, we cannot say today whether our solar system – composed of a combination of giant, gaseous and small, rocky planets – is typical or a rare exception.

More importantly, we cannot establish whether planets potentially hosting life are frequent or not. The vast majority of planets that we have detected so far are hostile to life, at least to the kind we are familiar with. Most of the 5,000 planets identified so far are either too hot or cold to host life, or lack any atmosphere. Only a few of them are likely covered by a large ocean of water that may have the conditions to support life – despite some tantalizing results [5], there is no solid evidence of the existence of some form of life in these planets (see Figure 3).

While this picture is largely due to the limitations of our current telescopes, which cannot yet identify earth-like planets, we can nevertheless conclude that the Universe is largely hostile to life. While we cannot, of course, exclude that other inhabited planets exist, among the billions of billions of stars that fill the Universe, we can safely conclude that recent results support the belief that our planet is rare enough in the Universe – a further good reason to take care of our fragile planet, which is a rare gem of life in a hostile Universe.

The History of the Universe

The other central question that inevitably comes to mind is: “How did all the Universe start?” This question has “tormented philosophers”, as Galileo wrote, since the beginning of mankind. Until the late 60s, astronomers have debated between two different ideas: the static Universe, built on the idea of an eternal Universe where small quantities of matter were forming spontaneously to keep creating new stars and planets; and the Big Bang scenario, stipulating that the Universe started a finite amount of time ago, and has kept expanding since then. According to this scenario, the Universe was initially filled only with primordial gas, from which stars and galaxies formed later and are still forming today. Again, the controversy between the two theories has been settled thanks to more powerful telescopes that have been used as time machines, literally looking backward in time. Indeed, the light that we observe from objects that are very distant from us has traveled for a long time before reaching our telescopes. For this reason, the light that we see today has been emitted a long time ago, and reveals how the object was at that time, not today. Modern telescopes can observe incredibly distant galaxies, even more than 13 billion light-years from now (see Figure 4). This way, our telescopes reveal how these objects were when the light was emitted, not how they are today. By looking at more and more distant galaxies, we have discovered that galaxies started to form about 13.5 billion years ago, as tiny clouds of stars thousands of times smaller than today, and that they have grown over time changing shape and size – pretty much like living species do. An example is shown in Figure 5. Other telescopes have stretched our vision even further, detecting the primordial gas that filled the Universe before the first stars were born. These and other crucial evidence demonstrate that the Universe has an history, which started about 13.7 billion years ago in an extremely dense phase that we call Big Bang, and has been expanding ever since. Thanks to these discoveries, the Big Bang scenario is not a controversial theory anymore, but an established fact that is considered one of the most important discoveries of modern science.

As undisputable as this evidence is today, it opens other fundamental questions that are still unanswered. What really was the Big Bang? Is there any creation event involved in this process, or does it stem from ordinary physical laws? Did anything exist before the Big Bang? Will the Universe continue expanding forever? What is the ultimate fate of the Universe and everything it contains?

This a realm where several theories have been formulated but no clear evidence has been established. Some of these theories even speculate that other Universes may have existed before or possibly in parallel with our Universe, maybe regulated by entirely different laws of physics. It is tempting to imagine that, after discovering that the Sun is a star among billions of stars, and the Milky Way is a galaxy among billions of galaxies, we will one day prove that the Universe itself is one among billions of Universes – but so far, this is just a speculation arising from the imagination and creativity of scientists.

What is the Universe made of?

One of the reasons why we are unable to understand the nature of the Big Bang is that the Universe seems to be made of more “ingredients” that we can naively imagine. We know that ordinary matter – that is, the same kind of atoms that we see in our Earth and Solar System – makes up stars and planets in the Universe.

However, we have good reasons to believe that something is missing in the picture. Indeed, we see that the expansion of the Universe and the motion of galaxies in the Universe seem to be driven by something much more powerful than the gravitational force, due to the matter that exists in the Universe. The simplest explanation is to postulate the existence of two new “ingredients”. One is a kind of matter that we have never seen on the Earth or in our laboratories. It must be a kind of matter (likely made of unknown particles) that is as much as six times more abundant in the Universe than normal matter, yet it does not interact in any measurable way with ordinary matter. For its elusive nature we call it Dark Matter.

In addition, we have proven not only that the Universe is expanding, but that it is expanding faster and faster: something seems to be pushing and inflating the Universe. We postulate the existence of an unknown form of energy that drives the expansion. As a whole, this energy must exceed by several times all other sources of energy in the Universe, summing up to at least ¾ of the total energy of the Universe. To express the scientist’s dismay in front of this puzzling discovery, they have named it Dark Energy.[1] Other explanations exist, that postulate a modification to the fundamental laws that describe the nature and effect of the gravitational force but are apparently unable to describe all the observational evidence.

Whatever the explanation of our observations is, they demonstrate that we are still lacking a profound understanding of the ultimate nature of the Universe. Just when we thought we had grasped its ultimate composition, we must conclude that the Universe is still a mystery that exceeds our current comprehension.

Are observations of the Universe at risk?

To solve all these mysteries, and maybe discover others, scientists keep developing new telescopes and new techniques to observe the Universe farther and deeper. However, a fundamental challenge is emerging that could ultimately make this effort more difficult: the development of “mega-constellations” of low-orbit satellites for internet broadcasting (see Figure 6).

Over the last few years, flocks of low-cost, small-size satellites have been launched to provide internet coverage across the entire globe, especially on land and sea areas that cannot be reached by ordinary wire or cellular connection. The most effective company to develop this service (Space X) has become world famous and already provides users with global coverage employing only a few thousands of such satellites.

Future plans of Space X and other companies, based in several countries, foresee the development of dozens – and possibly hundreds – of these satellites in a few years.

The impact of these satellites on astronomical observations is potentially severe. In the optical light, where our eye and ordinary telescopes are sensitive, they inevitably reflect the Sun’s light especially after sunset and before dawn, when they are high enough to be illuminated by the Sun even if on the Earth it is already dark. Being bright enough to be seen even with the naked eye, they are easily detected by modern telescopes and affect their observations substantially. In particular, observations of large patches of the sky at the beginning or at the end of the night, which are typical, for instance, of observations that target asteroids close to the Sun and the Earth, are the most severely endangered.

Radio telescopes, which observe in regions of the radio spectrum so far preserved for astronomy, are at even greater stake. These satellites are powerful sources of radio emission, which they use to broadcast internet signals. This signal can be so powerful as to saturate the entire signal detected by radio telescopes even at the radio frequencies preserved for astronomy.[2]

While astronomers acknowledge the economic and social importance of developing a distributed access to the internet across the globe, they are obviously concerned by the potential impact of these systems. Not only they may affect multi-billion-euro projects for new telescopes which are in development, but they can eventually fundamentally limit our future capability of observing the sky from the Earth’s surface.

For this reason, mitigation measures are being discussed with the satellite companies to alleviate their impact, but these are based on and limited by a best-effort, good-will approach on both sides, given the lack of international regulation on the matter. High-level discussions at the international level are ongoing. The International Astronomical Union and other agencies are actively promoting global awareness, in the context of the broader issue of the preservation of dark skies.

It is impossible, though, not to see the broader impact that these satellite constellations will have on many populations, for which the observation of the sky is an essential ingredient of their vision of life and a crucial input to take informed decisions on their activities. Should these constellations develop by orders of magnitude, as the current plans foresee, they will represent a substantial form of pollution of the last untouched environment: the deep sky.

Bibliography

[1] Galileo Galilei, 1610, Sidereus Nuncius.

[2] Hubble, E. 1938, Publications of the Astronomical Society of the Pacific, Vol. 50, No. 294, p. 97

[3] Mayor, M., Queloz, D., 1995, Nature, 378, 355.

[4] https://www.nasa.gov/universe/exoplanets/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18-b/

[5] Madhusudhan, N. et al., 2023, The Astrophysical Journal Letters, 956,13

[6] https://science.nasa.gov/mission/webb/

[7] Papovich, C., et al., 2015, The Astrophysical Journal 803, 26

[1] Some claim that this dull name more effectively proves the scientists’ lack of fantasy, actually…

[2] Future generation radio telescopes will be sensitive enough to detect the (incredibly faint) radar emission potentially originated by earth-like airports situated on planets distant up to 70 light-years from the Earth, if they exist. The effect of a powerful radio emitter flying only 500km above the telescope can easily be imagined.