Classifying the Cosmos: How We Can Make Sense of the Celestial Landscape

By Steven J. Dick

Since the invention of the telescope 400 years ago, astronomers have rapidly discovered countless celestial objects. But how does one make sense of it all?

Astronomer and former NASA Chief Historian Steven J. Dick brings order to this menagerie by defining 82 classes of astronomical objects, which he places in a beginner-friendly system known as "Astronomy’s Three Kingdoms.” Rather than concentrating on technicalities, this system focuses on the history of each object, the nature of its discovery, and our current knowledge about it.

The ensuing book can therefore be read on at least two levels. On one level, it is an illustrated guide to various types of astronomical wonders. On another level, it is considerably more: the first comprehensive classification system to cover all celestial objects in a consistent manner.

Accompanying each spread are spectacular historical and modern images. The result is a pedagogical tour-de-force, whereby readers can easily master astronomy’s three realms of planets, stars, and galaxies.

• Language: English
• Publisher: Springer
• Release date: Mar 21, 2019
• ISBN: 9783030103804

Book preview

Part I

The Kingdom of the Planets

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Montage of planetary images taken by NASA spacecraft, ranging from Mercury at top to Neptune at bottom. Credit: NASA/JPL

The Kingdom of the Planets is the realm of astronomy most familiar in popular culture. This largely has to do with the fact that its objects are closest to our home planet and thus are most easily observable both from the ground and—uniquely among the Three Kingdoms—from in situ spacecraft. If Earth is our first celestial address, Solar System is the second, and we have been surveying it since the invention of the telescope more than 400 years ago. We now know that it harbors not only classes of large objects such as planets, satellites, and rings, but also smaller objects such as asteroids, meteoroids, and comets, as well as the gas and dust of the interplanetary medium—not to mention energetic particles that whiz through the system from a variety of sources including its central Sun. And only in the last quarter century have we empirically discovered what was long conjectured to be the case: that the Solar System is not alone, that almost all stars have planets, many of them in planetary systems both similar and diverse compared to ours. The close-up scrutiny of our Solar System therefore tells us something about systems that exist around stars throughout the universe—but only to a point.

The discovery of our Solar System is a continuous process extending even until today. Five planets beyond Earth were known since ancient times, wandering as they were among the seemingly fixed stars. William Herschel added Uranus in 1781, Johann Galle clinched Neptune in 1846, and Clyde Tombaugh picked out the slow motion of Pluto in 1930 amidst thousands of other faint background objects. Gas giant planets like Jupiter were distinguished from terrestrial planets like Earth only in the 19th century, and ice giants only in the late 20th. And in an indication that discovery never ends as new details are revealed, in 2006 the International Astronomical Union controversially declared Pluto a dwarf planet, a new class that was officially different from a planet, leaving the Solar System with only eight, much to the chagrin of the public and some astronomers. Beyond our Solar System, the true nature of the thousands of exoplanets in other stellar systems remains for the most part unknown, while the pulsar planets first detected in 1992 are so different that they deserve a class of their own. One thing was for sure: the discovery and classification of exoplanets (P 18) would remain a major activity for astronomers in the 21st century. Not only that, but new techniques in the 21st century, particularly at infrared, millimeter, and submillimeter wavelengths, are unveiling the birth of planets in the form of protoplanetary disks around other stars.

Meanwhile in the circumplanetary Family (Chapter 3), satellites, rings, and radiation belts had their own stories of discovery. Our Moon was a class of one until Galileo’s discovery of the four moons of Jupiter in 1610. By 1950, only 30 satellites were known, but today they number at least 187, including 12 discovered around Jupiter in 2018 alone. The incredible diversity of all these satellites has been a major source of study, and astronomers now appreciate satellites and possible exomoons as objects of study in themselves, and even as possible locations for life. Galileo discovered the rings of Saturn in 1610, a new class of objects that we now know to be common around gas giant planets. The discovery of the Van Allen radiation belts at the beginning of the Space Age completed the repertoire of known circumplanetary objects, a class soon discovered around some of the other planets as well.

In the subplanetary Family (Chapter 4), dwarf planets and meteoroids (in the form of meteor showers and storms) are often in the news. Those objects now officially designated small bodies of the Solar System have also drawn attention throughout history, especially in the form of those spectacular comets that visit the Earth, if only briefly. The first asteroid, Ceres, was discovered on New Year’s Day 1801, sparking the beginning of a new class of objects that now number in the tens of thousands. As we will see, in another story of classification, Ceres has now been declared a dwarf planet. Why? You will find out in entries P 9 and P 11.

Asteroids are not just distant objects of little concern to the citizens of Earth: near-Earth objects (NEOs) are the subject of close scrutiny for their potential to strike the Earth and cause catastrophic damage. Finally, beyond the terrestrial and giant planets is what has been called the third zone of the Solar System, a zone we now know is filled with objects generally classed as Trans-Neptunian, some of which gather into Trans-Neptunian systems such as the Kuiper Belt and the Oort Cloud.

Not to be forgotten are objects even smaller than these subplanetary objects, composing the interplanetary medium Family (Chapter 5) of gas, dust , and energetic particles emanating from the solar wind and from the more distant regions of interstellar space. Though invisible to the human eye, they play an important part in the planetary Kingdom and the ecology of the Solar System.

Finally, as we have hinted in the case of the Kuiper Belt and the Oort Cloud, and as our planetary system itself attests, astronomical objects tend to gather together into systems under the force of gravity. These systems include not only well-known asteroid groups such as the main belt between Mars and Jupiter, but also many other asteroid conglomerations such as the Trojans, Centaurs, and near-Earth asteroids (NEAs). Similarly, meteoroids exist not only at random, but also tend to gather in meteoroid streams, most often because they are the remnants of comets that intersect the Earth’s orbit, causing sometimes spectacular meteor showers or meteor storms.

The Kuiper Belt and the Oort Cloud, the third (and perhaps the fourth) zones of the Solar System, extend the Sun’s influence far beyond the planets we usually think of as the outer boundaries of the system. They contain object believed to be leftovers from the Solar System’s formation, which are occasionally propelled toward Earth in the form of comets.

Altogether, the 22 classes of objects we designate in the Kingdom of the Planets make it a busy and interesting place, an object of continuous study and a source of amazement to those who take the time to contemplate Earth’s immediate surroundings.

© Springer Nature Switzerland AG 2019

S. J. DickClassifying the CosmosAstronomers' Universehttps://doi.org/10.1007/978-3-030-10380-4_1

1. The Protoplanetary Family

Steven J. Dick¹ 

(1)

Ashburn, VA, USA

Class P 1: Protoplanetary Disk

All things must be born, and for planets birth begins with a protoplanetary disk, a circumstellar disk of gas and dust that originates during star formation, when a protostar (S 1) condenses out of a molecular cloud (S 25). The central star is believed to be a T Tauri or higher mass object known as a Herbig Ae/Be star (S 2 and S 3) where planets have not yet had time to accrete. Recent research indicates that in massive starburst clusters such as those in the Carina nebula , the ages of these pre-main sequence stars varies from half a million to 20 million years. Protoplanetary disks are distinguished from debris disks (S 15) left over after planet formation, which may be equivalent to (or younger versions of) the Kuiper belt (P 21) in our Solar System. One form of protoplanetary disk is known as a proplyd , an externally illuminated photoevaporating disk first observed in the Orion Nebula . Proplyds represent what our Solar System must have looked like at an age of about one million years. The lifetime of protoplanetary disks is not expected to exceed several tens of millions of years before the formation of kilometer-sized planetesimals and then planets. Not all disks will result in planets; simulations have shown that rings may arise around stars that do not form planets.

It had long been theorized since Laplace’s nebular hypothesis in the 18th century that a circumstellar cloud of material was a necessary part of star formation. Prior to the 1980s, the existence of protoplanetary disks was inferred from such theories, as well as models of the protosolar nebula. The challenge was empirical confirmation. Observations reported in 1979 indicated the presence of compact ionized sources around some stars, and by 1987 one of the possible models of these sources was a circumstellar shell around young stars. Meanwhile in the mid-1980s, the Infrared Astronomical Satellite (IRAS) found infrared excesses around some T Tauri and main sequence stars, indicating the presence of cold circumstellar dust. Many of these disks were erroneously interpreted as protoplanetary. Only later were the class of protoplanetary disks separated from debris disks , the former containing more dust and still accreting onto the central young star, the latter containing less dust as well as planets and planetesimals . Different types of protoplanetary disks may exist depending on the environment in the star-forming cluster.¹

Hubble Space Telescope observations first proved the existence of protoplanetary objects in the form of proplyds (Fig. 1.1). In 1992 the astronomer C.R. O’Dell and his colleagues at Rice University were making observations to study the fine structure of the Orion nebula when they unexpectedly found the circumstellar disks, previously thought to be stars, but now rendered visible to Hubble by being near a hot ionized cloud of hydrogen known as an H II emission nebula (S 24). We knew that such circumstellar clouds are a necessary part of the formation of new stars, O’Dell recalled. We also knew that the Orion Nebula Cluster was very young. What we had not expected was that these circumstellar clouds would be rendered easily visible because of the Orion Nebula. With all the wisdom of hindsight, we now understand that this must be the case, and we should have been planning special observations just to detect the proplyds . Instead, this was a good example of serendipity in science.²

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Fig. 1.1.

Protoplanetary disks in the Orion Nebula , captured by the Hubble Space Telescope , are believed to be fledgling planetary systems . Only six of many are highlighted here. Credit: NASA /ESA/Hubble

O’Dell coined the term proplyd to describe these extended disks of dust around 15 newly formed stars observed with the Hubble Space Telescope in Orion. The name (from PROtoPLanetarY DiskS) was created following the suggestion of O’Dell’s wife, Gail Sabanosh, who joked that for him the phrase protoplanetary disks was too much of a tongue twister. The Hubble press release, which touted the objects as the strongest evidence yet that many stars form planetary systems, quoted O’Dell as saying these young disks signify an entirely new class of object uncovered in the universe.³

Not everyone agreed that these were indeed protoplanetary disks. But by 1994, O’Dell and his colleagues had found 56 proplyds around 110 stars in Orion and demonstrated that they are likely pancake-shaped disks of dust rather than shells of dust—further evidence of their protoplanetary nature. By 1996 they announced 145 compact sources that can be classified as proplyds.⁴ Depending on the perspective and the illumination conditions, the circumstellar material is sometimes seen as a bright-rimmed object, and other times as a dark object silhouetted against a bright background. An Atlas of protoplanetary disks in the Orion Nebula published in 2008 contained 178 proplyds (defined as externally ionized protoplanetary disks) and 28 silhouette disks, defined as disks seen only in absorption against the bright nebular background." Several other types, also distinguished by physical appearance, are evidence of a new field still in great flux.⁵

The Orion star-forming region was not alone. In 2003, astronomers Nathan Smith, John Bally, and colleagues announced the discovery of numerous candidate proplyds in the Carina nebula , an H II region like Orion, some 7,300 light years distant. Their discovery comes as a surprise because Carina is powered by much hotter and more massive stars, including eta Carina , one of the most massive stars known. It was thought such stars would have blown away the proplyds , or at least given them much shorter lifetimes. The Carina proplyds were considerably large than those in Orion and were labeled candidate proplyds because of the possibility that they were remnants of the molecular cloud and not protoplanetary in nature, in which case they would be interesting examples of ongoing star formation. Since their initial discovery, protoplanetary disks have also been imaged in the Carina Nebula by the Atacama Large Millimeter/submillimeter Array (ALMA ).⁶ The observations were made using the 4-meter telescope of the Cerro Tololo Inter-American Observatory (CTIO) in Chile.

Proplyds observed thus far in the Orion Nebula vary from 100 to 1000 astronomical units across; by comparison, our Solar System out to Pluto is 40 astronomical units. Those in Carina are up to ten times larger—100 times the diameter of the Solar System. They are associated with the stellar jets and Herbig-Haro Objects (S 20 and S 21) that come with starbirth. Spectra taken with the Spitzer Space Telescope have indicated the first building blocks of planets and life embedded in protoplanetary disks, including silicates and water, methanol, and carbon dioxide ice.

The next great step in imaging protoplanetary disks came with the construction of ALMA at an altitude of 16,500 feet in the Chilean Atacama desert. This amazingly innovative instrument began imaging these disks even in the midst of its testing and verification process in 2014. The 66 high-precision dish antennas of ALMA are ideal for directly imaging the formation of planets at millimeter and submillimeter wavelengths. The image released in 2014 shows a young star known as HL Tau (Fig. 1.2) surrounded by a disk comprised of multiple rings and gaps as the emerging planets sweep their orbits clear of dust and gas—a striking confirmation of the nebular hypothesis. The system, located about 450 light years from Earth, is less than one million years old. ALMA has since imaged many such systems of various ages, including the Orion proplyds and in 2018 the object known as AS 209 , located 410 light years away in the Ophiuchus star forming region. In the same year, astronomers using ALMA detected three Jupiter -mass planets still forming—actual protoplanets rather than fully formed ones. They did so by detecting unusual patterns in the flow of carbon monoxide gas in the protoplanetary disk surrounding HD 163296. That system is about 4 million years old and is located about 330 light years from Earth in the direction of the constellation Sagittarius.⁷

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Fig. 1.2.

Protoplanetary disk surrounding the young star HL Tau . Credit: ALMA (ESO/NAOJ/NRAO)

Other ground-based telescopes have also captured protoplanetary disks, including the SPHERE instrument at optical wavelengths on the Very Large Telescope in Chile’s Atacama desert. The project released images of three planet-forming disks in 2016. The instrument, an acronym for Spectro-Polarimetric High-contrast Exoplanet Research, uses a coronagraph to block light from the young stars to detect the surrounding disks. In another first, in 2018 astronomers using SPHERE directly imaged a planet still forming around its star. The parent star was a five to ten-million-year-old T Tauri or dwarf star known as PDS 70, 370 light years from Earth in the constellation Centaurus. The planet, named PDS 70b following the usual nomenclature, has an upper mass limit of 5–14 Jupiter masses, a temperature around 1,000° C, and is at a distance of 20 astronomical units from its parent star, giving it an orbital period of around 120 years.⁸

The discovery of circumstellar disks in the 1980s and proplyds in the 1990s still begged the question of whether planets would eventually form to comprise planetary systems (P 18). The answer was not long in coming. The first unambiguous planets around Sun -like stars were discovered beginning in 1995, and thousands are now known. ALMA and other telescopes are now advancing our knowledge of the stages before these planets were formed. Surprisingly, studies of interplanetary dust (P 15) are also adding to our knowledge of the formation of planetary systems.

More images and information on protoplanetary disks is available at the Hubble Space Telescope, Spitzer Telescope, and ALMA sites. For a Hubble Atlas of 30 protoplanetary disks in Orion see https://​www.​spacetelescope.​org/​news/​heic0917/​; for Spitzer http://​spitzer.​caltech.​edu/​search/​image_​set/​20?​search=​protoplanetary&​x=​11&​y=​10; and for ALMA http://​www.​almaobservatory.​org/​en/​home/​. A European Southern Observatory video zooming in on the planet forming around PDS 70 can be found at: https://​www.​youtube.​com/​watch?​v=​y8nRsiAK92Y

Footnotes

1

Two early reviews of this subject with history are Aki Roberge and Inga Kamp, Protoplanetary and Debris Disks, in Sara Seager, ed., Exoplanets (Tucson: Univ. Arizona Press, 2010), and Jonathan P. Williams and Lucas Cieza, Protoplanetary Disks and their Evolution, ARAA, 49 (2011), 67–117. A more recent review is Quentin Kral et al., Circumstellar Discs: What Will be Next?, in Hans Deeg and Juan Antonio Belmonte, eds., Handbook of Exoplanets (New York: Springer, 2017).

2

C. R. O’Dell, The Orion Nebula: Where Stars are Born (Cambridge, MA: Harvard University Press, 2003), pp. 131–132. The discovery paper is C. R. O’Dell, Zheng Wen and X. Hu, Discovery of New Objects in the Orion Nebula on HST Images – Shocks, Compact Sources, and Protoplanetary Disks, ApJ, 410 (1993), 696–700.

3

HST Release, NASA’s Hubble Space Telescope Discovers Protoplanetary Disks Around Newly Formed Stars, Dec. 16, 1992, http://​hubblesite.​org/​newscenter/​archive/​releases/​1992/​29/​text/​

4

HST Release, Hubble Confirms Abundance of Protoplanetary Disks around Newborn Stars, June 13, 1994, http://​hubblesite.​org/​newscenter/​archive/​releases/​1994/​24/​text/​. C. R. O’Dell and Zheng Wen, Postrefurbishment Mission Hubble Space Telescope Images of the Core of the Orion Nebula: Proplyds, Herbig-Haro Objects, and Measurements of a Circumstellar Disk, ApJ, 436 (1994), 194–202; O’Dell, C. R., & Wong, S. K., Hubble Space Telescope Mapping of the Orion Nebula. I. A Survey of Stars and Compact Objects, AJ, 111 (1996), 846–855.

5

L. Ricci, M. Robberto and D. R. Soderblom, The Hubble Space Telescope/Advanced Camera for Surveys Atlas of Protoplanetary Disks in the Great Orion Nebula, AJ, 136 (2008), 2136–2151.

6

NOAO Release, Substantial Population of Stellar Cocoons Found in Surprisingly Harsh Environment, January 8, 2003, http://​www.​noao.​edu/​outreach/​press/​pr03/​pr0301.​html. Nathan Smith , John Bally , and Jon A. Morse, Numerous Proplyd Candidates in the Harsh Environment of the Carina Nebula, ApJ Letters, 587 (2003), L105–L108. The ALMA imagery is reported in A. Mesa-Delgado et al., Protoplanetary Disks in the Hostile Environment of Carina, ApJ Letters, 825 (2016), L 16. A few isolated proplyd-like objects have also been seen elsewhere, as in the Lagoon and Trifid Nebulae and in NGC 3603.

7

ALMA release, ALMA Discovers Trio of Infant Planets around Newborn Star, June 12, 2018; http://​www.​almaobservatory.​org/​en/​press-release/​alma-discovers-trio-of-infant-planets-around-newborn-star/​. A recent paper that gives some of the history and science of protoplanet detection is C. Pinte et al., Kinematic evidence for an embedded protoplanet in a circumstellar disc, http://​www.​eso.​org/​public/​archives/​releases/​sciencepapers/​eso1818/​eso1818a.​pdf

8

On PDS 70b see ALMA release, "First confirmed image of newborn planet caught with ESO’s VLT, June 2, 2018, https://​www.​eurekalert.​org/​pub_​releases/​2018-07/​e-fci062918.​php

© Springer Nature Switzerland AG 2019

S. J. DickClassifying the CosmosAstronomers' Universehttps://doi.org/10.1007/978-3-030-10380-4_2

2. The Planet Family

Steven J. Dick¹ 

(1)

Ashburn, VA, USA

Class P 2: Terrestrial (Rocky)

Once formed, planets come in a variety of classes that astronomers have distinguished over a period of several centuries. We are partial to the class of terrestrial planets because one of them is our home. In general, a terrestrial planet is one whose constituents are rocky and therefore similar to the Earth in composition, by comparison with the gas giant or ice giant planets (P 3 and P 4). By anyone’s definition, Mercury , Venus , Earth , and Mars are classified as the terrestrial planets in our Solar System. It is notable that these are the four inner planets of the system, certainly a clue to their formation