Planets Everywhere. Dave Stevenson Caltech. Insight Cruises, January 21, 2018

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1 Planets Everywhere Dave Stevenson Caltech Insight Cruises, January 21, 2018

6 Galileo s notebook (1610)

16 Detecting Planets by the Transit Method

19 Conclusion (So Far) Planets are common Most stars have at least one planet Many stars have planetary systems The most common kind of planet is more massive than Earth, less massive than Uranus or Neptune SuperEarths and subneptunes Two classes.. One stripped of atmosphere, the other with a hydrogen atmosphere Our solar system is not typical Earthlike planets may not be common

21 Why do Planets Exist? Angular momentum (a swirling motion of gas & dust) is everywhere When gravity causes collapse, a disk results Heavy elements exist Inevitable outcome of thermonuclear synthesis in stars These can condense (form rocks and ice); the resulting embryo can be the seed for a planet Except for the earliest times, planets should have formed everywhere and copiously

22 How to think about Planets? Could discuss provenance- the properties of an apple depend on the environment in which the tree grows Or could discuss it as a machine (cf. Hero[n], 1st century AD) Need to do both

23 Cosmic Abundances Hydrogen is about 73% of the (baryonic) mass in the Universe Hydrogen & helium are together about 99% of the baryonic universe. These are gases in planetary formation Elementary particles & Big Bang Oxygen is the third most abundant element in the Universe Add hydrogen to get water Oxygen, Carbon and Nitrogen (+ hydrogen) make Ices The rest (magnesium, iron, silicon) make Rock Planets are comprised of differing amounts of Gas, Ice and Rock

24 Solid Planets Terrestrial (silicates, oxides and iron alloy)- Mercury, Venus, Earth, Moon, Mars, Io Large icy satellites (terrestrial +ice) Europa, Ganymede, Callisto, Titan, Triton, Pluto

25 Fluid Planets Gas Giants (primarily hydrogen and helium)- Jupiter and Saturn Ice Giants (everything, but including large amounts of H 2 O at high P,T) Uranus and Neptune

26 Gas (H 2,He) Line of cosmic ice & rock condensate (variable gas) J,S U,N Ice (mainly H 2 O) Large Icy Satellites Earth Rock (silicates, oxides, met. Fe)

27 Gas sj subjupiters Not represented in our solar system Superganymedes Ice Rock J Extrasolars M/M J =10-4 LI Gas M/M J =10-2 SG E Increasing mass M/M J =1 Ice Rock

28 Is there a Planetary Equivalent of the HR Diagram? HR diagram is a 2D representation of much of stellar physics Diversity of planet composition makes it difficult to construct a planetary equivalent But it s even worse

29 Why do Planets of Similar Mass & Composition have different Behaviors? Distance from star plays a role (but not necessarily dominant) Planets (unlike stars) are made of thermodynamically complex materials Complex melting behavior (volcanism) Effect of alloying (e.g., sulfur in iron) Effect of water on rock rheology Role of chance in formation

30 Interstellar medium contains gas & dust that undergoes gravitational collapse A solar nebula forms: A disk of gas and dust from which solid material can aggregate

31 What should you Believe about Planet formation? Rapid collapse from ISM; recondensation of dust; high energy processing Small (km) bodies form quickly (<10 6 yr) [observation] Moon & Mars sized bodies may also form as quickly[theory]

34 Popular Concept of a Habitable Zone Goldilocks scenario Location, location, location! Size matters Also depends on availability of H 2 O. Very abundant in the Universe very under-abundant on Earth

35 Three Kinds of Oceans Earthlike Protected by a dense atmosphere (e.g.,greenhouse) Protected by ice

37 Planet (or spacecraft) headed for escape Jupiter (for example) Interstellar Planets? -they have (probably) been observed by gravitational lensing

38 Radiating surface at 34K (driven by radioactive heat alone) An Interstellar or Kuiper Belt Planet Physical surface at ~300K, an ocean! Dense (kilobar) H 2 atmosphere..optically thick at far IR

39 Nature 2011

40 Planet Earth - The Water Planet

41 Earth Habitability Our planetary habitat has all of the following: (a)physical and chemical conditions suitable for liquid H 2 O. (b) Possibility of sustained thermodynamic disequilibrium. (c) A very long ( geological ) period during which these conditions existed. (Infrequent killing events.) (d) Plate tectonics to provide a driver for change and diversity

42 Nature of Plate Tectonics Expression of mantle convection -how Earth loses heat Dictated by the brittle nature of near surface rocks and the ductile nature at depth

43 Plate Tectonics & the Role of Water Water lubricates the asthenosphere Water defines the plates Maintenance of water in the mantle depends on subduction; this may not have persisted except on Earth Plate tectonics helps the dynamo by promoting core cooling

44 Water Plate tectonics Magnetic field These all influence LIFE

45 The Unbiased View Energy sources, disequilibrium and liquid water are common The habitable zone is everywhere except the very hot places (e.g., inside stars) Our study of exoplanets should not be an obsession with Earthlike planets Diversity is expected and should be appreciated and explored SETI should be encouraged and we must persist though it may take a long time Going there is hard for us and getting here is hard for them We are living in an exciting time

Comparative Planetology I: Our Solar System

Comparative Planetology I: Our Solar System Guiding Questions 1. Are all the other planets similar to Earth, or are they very different? 2. Do other planets have moons like Earth s Moon? 3. How do astronomers