The 2019 Nobel Prize in Physics was shared between two astronomy discoveries.
Michel Mayor and Didier Queloz were awarded one half of the prize “for the discovery of an exoplanet orbiting a solar-type star.” Extra-solar planets, or exoplanets for short, are planets that are orbiting another star other than our Sun, and are therefore outside our Solar System. Way back in 1995, Mayor and Queloz discovered 51 Pegasi b – the aforementioned extra-solar planet. Until the 1990s it was widely assumed that other stars would also have planets orbiting around them (cf. Star Trek, Star Wars, and countless others, etc.) but there was no scientific observation to prove that, although theory supported the idea. Previously, in 1992, planets were confirmed orbiting the pulsar PSR B1257+12. Mayor and Queloz were the first to detect planets orbiting a main sequence star (a pulsar is among the various types of post-supernova remains of stars.)
Their discovery, and many more exoplanets detected since then, used measurements of the star’s radial velocity to detect its planet(s).
The radial velocity method of detecting planets around other stars is based on the laws of gravity: two bodies that are gravitationally bound together both actually orbit their common center of mass. In other words, a planet orbiting a star gravitationally pulls the star just as the star pulls it (per Newton’s Law of equal and opposite reactions, i.e. forces.) Because of the relative masses favoring the star by a very large factor, the star’s motion is very very small compared to the planet’s.
For example, picture just Jupiter, the largest planet in our Solar system and the Sun, orbiting their common center of mass. The force of gravity pulling both of them is equal, but Sun’s much greater mass makes it move much less, in keeping with Newton’s Third Law:
However, when all the planets are included, the picture – and the Sun’s motion – becomes much more complex. Whereas if we pictured only Jupiter and the Sun, they neatly orbited the center of mass, with many different planets pulling the Sun, all with different orbits and masses, the Sun’s motion would appear to be more of a wild spiral or wobble:
Notice in the diagram above that the size of the Sun’s motion is barely the same as its own size, and the path shown is over the course of decades! (like the orbits of the outer planets.) Obviously, this would be very hard to observe from another star’s distance, and at the very least would require decades of very precise observations.
Nevertheless, in recent years, astronomers have successfully detected the similar wobble of other stars! This has been accomplished using multiple methods.
One of these methods, astrometry, measures the visible “wobble” motion of a star against the sky’s background with extreme precision. As in the diagram above, for our Sun, from a distance of 30 light years (relatively close, by astronomical standards) the measurement would require precision of 0.0005 arcseconds! One arcsecond is one sixtieth of an arcminute, which is one sixtieth of one degree (of angle) – a tiny tiny angle! For reference, the angular size of the Sun and Moon, as viewed from Earth, are both about half a degree, or 30 arcminutes.
Another method, which has now won a Nobel Prize, measures a star’s planet-induced motion using the Doppler Method. Light from objects moving towards or away from an observer is called “Doppler shifted”; the frequency/wavelength/color of the light is shifted towards the red end of the spectrum for objects moving away from the observer and in the blue direction of the spectrum for objects moving towards the observer.
This motion can be detected by observing spectra of an object and detecting known absorption or emission lines of elements (or molecules…) that are shifted either towards red or blue. The amount of shift corresponds to the speed towards or away from the observer.
In the case of a star with a wobbling orbit due to one or more planets, this would cause the star’s spectrum to shift one way and then the other, over the course of the period of the orbit, in a periodic fashion corresponding to the star’s velocity.
Mayor and Queloz were the first to successfully use this method of doppler shift measurements to detect an extra-solar planet! Note that these are also extremely challenging and precise measurements, but of a different sort.
These methods are both particularly good at detecting massive planets that are very close to their stars, because then they can measure larger motions (due to more massive planets) in very short periods. Therefore, 51 Pegasi b, and many other early discovered exoplanets, fall into a category of previously unknown planet called “Hot Jupiters” that does not exist in our own Solar System. They are “hot” because they are very close to their stars – often closer than Mercury is to the Sun – and called “Jupiters” because they are very massive, like Jupiter. 51 Pegasi b, for example, has an orbital period of four days! Mercury, for comparison, has a period of about 88 days.
Hot Jupiters, when they were discovered, posed an interesting new puzzle in understanding the origins of planets. Prior to their discovery, based on our own Solar System as the only example, it was generally understood that Jupiter and other gas giant planets could only form in the farther-out region of the Solar System, beyond the “ice line” – where volatile molecules are frozen into icy particles. However, Hot Jupiters, due to their closeness to their stars, obviously seem to defy this explanation!
The answer to this puzzle is planetary migration!
Once Hot Jupiters were discovered – in abundance, after the first one – astronomers realized possible mechanisms that could cause planets in a forming solar nebula to migrate inwards towards their star. (Planets form in and from a disk of gas and dust around a star.)
One possibility is due to the planets’ own gravity creating denser waves in the proto-planetary disk around a star, as it gravitationally absorbs its surrounding “lane” of gas and dust. These denser waves then pull the young planet inwards.
Another possibility is due to two planets’ gravitational “close encounters” with each other or with smaller bodies (comets and asteroids.)
Due to transfer of energy and angular momentum in a close gravitational encounter between two massive planets, such an encounter can eject one planet while flinging the other into a highly elliptical orbit.
Repeated multiple close encounters with smaller planetesimals (e.g. comets & asteroids) can also cause inward migration, due to an exchange of angular momentum. Angular momentum is always conserved, so if one body, e.g. a comet, gets flung outwards from its initial orbit to a much larger orbit (due to the gravity of a planet) the planet loses some of its angular momentum to the comet, and moves inwards in its orbit.
Computer models of these processes have shown that they can successfully explain the previously puzzling locations of giant Hot Jupiters.
Planetary migration, once discovered regarding exoplanets, became an important part of understanding the formation of our own Solar System. Astronomers realized that effects like planetary migration and gravitational encounters might be more important than previously thought in the formation of our own Solar System too!
One series of research papers collectively called the “Nice Model” (after the city in France) has used simulations to show that the Jovian planets moved back & forth a lot, until they cleared their region of comets (which mostly ended up being flung out to the Oort Cloud and beyond.)
Now, what does this have to do with Bereishit – Genesis – as implied by the title of this post?
In most efforts to explain the Creation Narrative from Genesis chapter 1 in consonance with modern science, the order of the Sun, Moon and stars’ creation on the fourth day after the Earth itself on the second and third days seems fairly difficult to explain. While many explanations are possible, I will suggest one based on a midrash found in the Talmud, and related to the science discussed above:
ואור ביום ראשון איברי והכתיב ויתן אותם אלקים ברקיע השמים וכתיב ויהי ערב ויהי בקר יום רביעי כדר’ אלעזר דא”ר אלעזר אור שברא הקב”ה ביום ראשון אדם צופה בו מסוף העולם ועד סופו כיון שנסתכל הקב”ה בדור המבול ובדור הפלגה וראה שמעשיהם מקולקלים עמד וגנזו מהן שנאמר (איוב לח) וימנע מרשעים אורם ולמי גנזו לצדיקים לעתיד לבא שנאמר וירא אלקים את האור כי טוב ואין טוב אלא צדיק שנאמר (ישעיהו ג) אמרו צדיק כי טוב כיון שראה אור שגנזו לצדיקים שמח שנאמר (משלי יג) אור צדיקים ישמח כתנאי אור שברא הקב”ה ביום ראשון אדם צופה ומביט בו מסוף העולם ועד סופו דברי רבי יעקב וחכ”א הן הן מאורות שנבראו ביום ראשון ולא נתלו עד יום רביעי
b. Hagiga 12a
Was light created on the first day? Is it not written [Gen. 1:17]: “And God set them in the expansion of the heaven,” and also [ibid. 1:19]: “And it was evening and it was morning the fourth day”? This is as R. Elazar said: The light which the Holy One created on the first day, one could see by it from one end of the universe to the other. When the Holy One observed the generation of the flood and the generation of the dispersion, and that their actions were corrupt, He took it from them and concealed it from them, as it says “He kept from the wicked their light” [Job 38:15] And for whom did He hide it away? For the righteous in the world to come, as it says “And G-d saw the light that it was good” [Gen. 1:4] and there is no “good” except for the righteous as it says “Say [of] the righteous that [they are] good.” [Isaiah 3:10] When the light saw that it was hidden away for the righteous it rejoiced as it says “the light of righteous shall rejoice.” [Proverbs 13: 9]
In this, however, the Tanaim differ, as we have learned in a Baraitha: The light which the Holy One, blessed be He, created on the first day, Adam saw and observed [a person could see and observe] by its means from one end of the universe to the other. So said R. Jacob. But the sages said. These are the luminaries which were created on the first day, but were not hung up [in the heavens] until the fourth day.
The talmudic rabbis find a seeming discrepancy between verses about whether light(s) were created on the first or the fourth day of Creation.
Here we have a rabbinic explanation of the Genesis narrative that, according to the Sages’ opinion, says that the celestial lights – i.e. Sun, Moon and stars – were created earlier, on the first day! They were then, subsequently, placed in their current places in the sky. Where were they stored or located in between, according to this opinion? That is unknown or unexplained!
Planetary Migration can help explain this! (at least to some extent…) The planets, which were to the eyes ancient astronomers included in “the stars” (planets were “wandering stars” – but still stars…) actually did move from their original places of formation to their current orbits! (This is true for exoplanets as well, of course, even if the ancients had no knowledge of their existence and could not see them in the sky at all.) We could potentially understand this as corresponding to the opinion that heavenly bodies were created earlier than the fourth day of Creation, but moved to their current locations later in their histories. (Also, see my earlier post with respect to a similar idea about the timing of the Moon’s creation.)
Thus this Midrash about Bereishit is enlightened by the very modern understanding of planetary migration which resulted from the discovery of Hot Jupiter exoplanets, for which the recent Nobel Prize was awarded.