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Given the fact that Earth is so far the only place in the Milky Way galaxy known to harbor life, the question arises of whether the solar system is in any way special. To address this question, I compare the solar system to the many recently discovered exoplanetary systems. I identify two main features that appear to distinguish the solar system from the majority of other systems: (i) the lack of super-Earths, (ii) the absence of close-in planets. I examine models for the formation of super-Earths, as well as models for the evolution of asteroid belts, the rate of asteroid impacts on Earth, and of snow lines, all of which may have some implications for the emergence and evolution of life on a terrestrial planet. Finally, I revisit an argument by Brandon Carter on the rarity of intelligent civilizations, and I review a few of the criticisms of this argument.
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Representative abundances of the chemical elements for use as a solar abundance standard in astronomical and planetary studies are summarized. Updated abundance tables for solar system abundances based on meteorites and photospheric measurements are presented.
The measurement of the Suns diameter has been first tackled by the Greek astronomers from a geometric point of view. Their estimation of ~1800, although incorrect, was not truly called into question for several centuries. The first pioneer works for measuring the Suns diameter with an astrometric precision were made around the year 1660 by Gabriel Mouton, then by Picard and La Hire. A canonical value of the solar radius of 959.63 was adopted by Auwers in 1891. Despite considerable efforts during the second half of the XXth century, involving dedicated space instruments, no consensus was reached on this issue. However, with the advent of high sensitivity instruments on board satellites, such as the Michelson Doppler Imager (MDI) on Solar and Heliospheric Observatory (SoHO) and the Helioseismic and Magnetic Imager (HMI) aboard NASAs Solar Dynamics Observatory (SDO), it was possible to extract with an unprecedented accuracy the surface gravity oscillation f modes, over nearly two solar cycles, from 1996 to 2017. Their analysis in the range of angular degree l=140-300 shows that the so-called seismic radius exhibits a temporal variability in anti-phase with the solar activity. Even if the link between the two radii (photospheric and seismic) can be made only through modeling, such measurements provide an interesting alternative which led to a revision of the standard solar radius by the International Astronomical Union in 2015. This new look on such modern measurements of the Suns global changes from 1996 to 2017 gives a new way for peering into the solar interior, mainly to better understand the subsurface fields which play an important role in the implementation of the solar cycles.
Reinhold et al. (Science, 1 May 2020, p. 518) provided two possible interpretations of measurements showing that the Sun is less active than other solar-like stars. We argue that one of those interpretations anticipates the observed differences between the properties of their two stellar samples. This suggests that solar-like stars become permanently less variable beyond a specific evolutionary phase.
Passing stars can perturb the Oort Cloud, triggering comet showers and potentially extinction events on Earth. We combine velocity measurements for the recently discovered, nearby, low-mass binary system WISE J072003.20-084651.2 (Scholzs star) to calculate its past trajectory. Integrating the Galactic orbits of this $sim$0.15 M$_{odot}$ binary system and the Sun, we find that the binary passed within only 52$^{+23}_{-14}$ kAU (0.25$^{+0.11}_{-0.07}$ parsec) of the Sun 70$^{+15}_{-10}$ kya (1$sigma$ uncertainties), i.e. within the outer Oort Cloud. This is the closest known encounter of a star to our solar system with a well-constrained distance and velocity. Previous work suggests that flybys within 0.25 pc occur infrequently ($sim$0.1 Myr$^{-1}$). We show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. Using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer Oort Cloud is $sim$98%, but the probability of penetrating the dynamically active inner Oort Cloud ($<$20 kAU) is $sim$10$^{-4}$. While the flyby of this system likely caused negligible impact on the flux of long-period comets, the recent discovery of this binary highlights that dynamically important Oort Cloud perturbers may be lurking among nearby stars.
If the Sun was born in a relatively compact open cluster, it is quite likely that a massive (10MSun) star was nearby when it exploded in a supernova. The repercussions of a supernova can be rather profound, and the current Solar System may still bear the memory of this traumatic event. The truncation of the Kuiper belt and the tilt of the ecliptic plane with respect to the Suns rotation axis could be such signatures. We simulated the effect of a nearby supernova on the young Solar System using the Astronomical Multipurpose Software Environment. Our calculations are realized in two subsequent steps in which we study the effect of the supernova irradiation on the circumstellar disk and the effect of the impact of the nuclear blast-wave which arrives a few decades later. We find that the blastwave of our adopted supernova exploding at a distance of $0.15$--$0.40$,pc and at an angle of $35^circ$--$65^circ$ with respect to the angular-momentum axis of the circumsolar disk would induce a misalignment between the Suns equator and its disk to $5^circ.6pm1^circ.2$, consistent with the current value. The blast of a supernova truncates the disk at a radius between $42$ and $55$,au, which is consistent with the current edge of the Kuiper belt. For the most favored parameters, the irradiation by the supernova as well as the blast wave heat the majority of the disk to $sim 1200$,K, which is sufficiently hot to melt chondrules in the circumstellar disk. The majority of planetary system may have been affected by a nearby supernova, some of its repercussions, such as truncation and tilting of the disk, may still be visible in their current planetary systems topology. The amount of material from the supernova blast wave that is accreted by the circumstellar disk is too small by several orders of magnitude to explain the current abundance of the short live radionuclide $^{26}$Al.
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