Astrobiology Dr. Paul Wesselius 5 December 2018 Lecture 4: Exoplanets.

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1 Astrobiology 2018-2019 Dr. Paul Wesselius 5 December 2018 Lecture 4: Exoplanets

2 Contents Introduction Observing methods Kepler results Properties of exoplanets Atmospheres 2

3 INTRODUCTION 3

4 Literature Exoplanet Atmospheres, Seager, Princeton, 2010 Advances in Exoplanet Science from Kepler, Lissauer et al., Nature 513, 336, Sept 2014 Global Models Planet Formation & Evolution, Mordasini et al., Int. J. Astrobiology, 14, 201, 2015 Large exoplanet missions for 2020 Decadal Survey, 6 October 2015 Exoplanets, Summers & Trefil, Smithsonian books, 2017 The planet factory, Tasker, Bloomsbury, 2017 How to characterize the atmosphere of a transiting exoplanet, Deming et al, arXiv: 1810.04175, Oct 2018 (used in section on atmospheres) 4

5 Websites http://exoplanet.eu: The Extrasolar Planets Encyclopaedia http://exoplanet.eu http://exoplanets.org: California Planet Survey http://exoplanets.org http://planetquest.jpl.nasa.gov: JPL van NASA http://planetquest.jpl.nasa.gov http://www.planethunters.org: looking for exoplanets http://www.planethunters.org http://coolcosmos.ipac.caltech.edu/page/exoplanets 5

6 Names By voting, end 2015, names have been assigned to a number of exoplanets by the IAU (International Astronomical Union) The Netherlands (KNVWS) has given names to 55 Cancri and its 5 exoplanets: Copernicus, Galileo, Brahe, Lippershey, Janssen, Harriot 51 Pegasus and its exoplanet have been named by the Astronomische Gesellschaft Luzern: Helvetios and Dimidium (Celtic tribe and ‘half’, half as heavy as Jupiter) See: http://nameexoworlds.iau.orghttp://nameexoworlds.iau.org 6

7 Classification In 2006, the IAU has defined a planet in our own solar system as follows: – Describes an orbit around the sun, – Is massive enough to adopt a spherical shape, – Has cleared its orbit of asteroids Definition can be applied to exoplanets as well: stellar mass, planet mass and average distance star-planet can be determined  it can be derived whether the exoplanet has - plausibly - cleaned its orbit Nevertheless: new definition will come 7

8 8 http://www2.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html (The Advanced Light Source (ALS) is a research facility at Lawrence Berkeley National Laboratory in Berkeley, California. They have one of the world's brightest sources of ultraviolet and soft x-ray light.)

9 Spectroscopy 9 http://www.4college.co.uk/as/el/spec.php

10 Radial velocity 10 https://cseligman.co m/text/stars/radialve locity.htm

11 Spectroscopy of stars 11 http://www.drjackie.freeservers.com/PLANETARIUM/PAGE5_spectro_2.html

12 Planet orbiting 51 Pegasi At a distance of 48 light years a planet, somewhat smaller than Jupiter, orbits 51 Peg, in only 4 days (!!). It takes our Jupiter 12 years to orbit our Sun. Observational accuracy reached was 1 in 10 miljoen!! Fase 12 Snelheid (m/s)

13 Discovery of exoplanets The discoverers of the planet orbiting 51 Peg, in 1995: Michel Mayor and Didier Queloz Geoff Marcy and Paul Butler expected ‘Jupiters’ with orbital periods of order 12 years Mayor and Marcy got the ‘Shaw’ prize of $ 1 million, in 2005, in Hongkong 13

14 OBSERVING METHODS 14

15 Barycentre Sun is 330,000 times heavier than the Earth Sun and Earth move around a common centre of gravity, the barycentre Barycentre Earth-Sun lies at 1 ‰ from the centre of the Sun Thus, to a good approximation the Earth moves around the solar centre However, also the Sun moves – around this point at 1 ‰ from its centre Such minuscule movements can be measured in the line of sight (radial velocities) or perpendicular to the line of sight (astrometry) 15

16 Doppler effect Light waves from a source moving towards or away from the observer experience the Doppler effect resulting in either a red shift or blue shift in the light's frequency This is in a fashion similar to e.g. sound waves; the major difference is that light waves do not require a medium, they move through vacuum For the exoplanets the velocities are so low that the simple formula / = v/c is valid We have to measure relatively small velocities of only 0.1 to maximally 30 m/sec Compare to the speed of light: 300 million m/sec An accuracy of at least 1 in 10 million is needed!! 16

17 17

18 18

19 HARPS, from Mayor, at ESO Even 0.5 meter/second velocity accuracy has been reached for Proxima Centauri ( 1 w.r.t. 600 million!!) 19

20 ESPRESSO Espresso is an echelle spectrograph that is installed on all four VLT telescopes and can combine the light from these four telescopes - simulating a 16 meter diameter telescope It has done its first observations in autumn 2017 It can – in principle – reach accuracies of a few cm/sec, significantly better than HARPS Science verification phase in February 2019 Espresso can also be used for other studies 20

21 Astrometry Astrometry concerns precise measurements of positions, and movements over the sky, of celestial bodies (mostly stars) By combining astrometric and radial velocity measurements, information on the kinematics and physical origin of our galaxy, the Milky Way, are obtained The necessary accuracy of 1 in 10 million, also for astrometry, was mostly unobtainable until the advent of the satellite Gaia, operating since December 2013 On 25 April 2018 Gaia data release 2 contained information on ~1.3 billion stars 21

22 Via Astrometry 22 http://nccr- planets.ch/faqs/how- do-we-search-and- find-exoplanets-2/ Star at 50 pc, proper motion 50 milli arc seconds per year, planet of 15 M Jupiter, is 0.6 AU away from its star and has eccentricity of 0.2. Perryman (2015) estimates that 21,000 long-period planets of 1-15 M Jupiter will be found by Gaia.

23 Transits 23 From: Kepler NASA website

24 Transit animation 24 See also: https://www.youtube.com/watch?feature=player_embedded&v=RrusIZaWDW8

25 25 Planet transit over HD209458 ( Hubble Space Telescope ) Fenomenal accuracy Decrease in Intensity

26 Gravitational lens 26 http://flynt.pbworks.com/w/page/9198343/gravitational%20lensing%2 0or%20microlensing

27 Exoplanet and microlensing 27 http://blogs.scientificamerican.com/life-unbounded/gravitational-mesolensing-and-the- hunt-for-exoplanet/ Red lens star exactly transits background star and brightens its light A planet orbiting the lens star extra brightens the background star light

28 Gravitational lens brightening http://www.nature.com/nature/journal/v439/n7075/fig_tab/ nature04441_F1.html 28

29 29 Detecting planets near a star is like viewing a firefly near a lighthouse Direct observations

30 30 The firefly

31 Infrared observations infrared 1 billion 1 million 31 Intensity Difference in intensity is ‘only’ one million in infrared

32 New satellites Many radial velocity and transit observations from the ground; often automatic; especially bright stars TESS (NASA): launch 18 April 2018, 0,5 million stars; first data: early December CHEOPS (ESA): launch in window 15 Oct – 14 Nov 2019, study known stars with exoplanets in more detail PLATO (ESA): launch 2026, study of planetary transits and oscillations of bright stars  mass and radius of a star is much better known  also planetary mass and radius can be determined a lot better 32

33 Plato concept 33 Plato will have an array of 34 telescopes mounted on a sun- shield. It will monitor thousands of stars looking for the minute dip in light caused by the passage of a planet in front of its parent star. The combined light of dozens of cameras allows 5% of the sky to be monitored to large accuracy at any one time. See: https://lostintransits.wordpress.com/2014/01 /30/what-can-plato-do-for-exoplanet- astronomy/ https://lostintransits.wordpress.com/2014/01 /30/what-can-plato-do-for-exoplanet- astronomy/

34 James Webb In 2021 the James Webb Space Telescope will be launched towards L2. Its 6,5 meter diameter mirror will open a new era for space observations. 34

35 ELT instruments The Extremely Large Telescope, diameter 39 meter, of ESO, is planned to be operational in 2024. Four instruments have been approved: – MAORY, adaptive optics (a.o. for MICADO), – MICADO: near infrared, – HARMONI: spectrograph for many directions together, – METIS: mid infrared Also EPICS is studied, to observe exoplanets directly All these instruments can be used to observe exoplanets 35

36 KEPLER RESULTS 36

37 Kepler history Already in 1984 Borucki began to prepare Kepler In 2000 he made the fifth proposal, accepted in 2001 Launch on 6 March 2009, worked as specified until mid-2013; observations of a field in Cygnus A limited follow-up mission lasted until 30 October 2018 2327 (+ 355 K2) exoplanets have been discovered by Kepler; 2426 (+ 473 K2) wait for confirmation https://exoplanetarchive.ipac.caltech.edu/docs/co unts_detail.html https://exoplanetarchive.ipac.caltech.edu/docs/co unts_detail.html 37

38 Kepler fields 38

39 Kepler observations ~ 100,000 stars, once per 30 minutes during 4 years Precision: 30 in a million for 12th magnitude stars In Cygnus region a transition of the Earth over the Sun would take 13 hours, and cover a 80 millionth part of the solar disk 3600 potential exoplanets found in first three years with orbital periods usually shorter than a few months Most Kepler planets have a radius between 1 and 3.8 R earth Such planets do not exist in our solar system (!!) 39

40 Sizes of Kepler planets

41 50<P orb <300 day (Burke et al. 2014) 41 True distribution planets

42 Planets around Kepler 62 & Sun 42 http://www.nasa.gov/content/kepler-62-and-the-solar-system

43 Kepler Statistics At least as many planets as stars One in ten stars has at least one planet similar in size to the Earth Half the Kepler planets occur in multiple planet systems Such planets orbit in general in one plane and rather close to each other From known radius and mass of a star and the Kepler observations of a transiting planet  radius planet and distance to the star (Kepler’s third law) Disturbances in the regularity caused by multiple planets around one star can be measured  they are real planets 43

44 PROPERTIES OF EXOPLANETS 44

45 45 http://phl.upr.edu/projects/habitable-exoplanets-catalog The catalogue mentions 55 exoplanets that might be habitable and presents a list of the 14 best candidates (see figure, 21Nov18).

46 46 Radius exoplanets (in earth radii) Mass exoplanets (in earth masses) Red spheres are exoplanets When radius and mass are known the density can be calculated. ‘Rocky planets’: Rock (3), iron (8), average ~ 5 ‘Ice planets’: consist completely of water (mostly as ice) ~ 1 ‘Jovian planets’: H/He and a solid nucleus of at most 10% of the mass From: Mordasini et al. 2015

47 ATMOSPHERES EXOPLANETS 47

48 Observing atmospheres Studying an exoplanet – far enough away from its parent star – separately, and making a spectrum would be ideal That has to wait for future instrumentation like the JWST and the ELT Fortunately, exoplanets transiting over their parent star – for the time being Jupiter-sized exoplanets very close to the parent star – allow already to study the atmospheres of such exoplanets 48

49 Atmospheres and transits 49 Atmospheres of transiting exoplanets can be studied in several ways.

50 Fundamental principles Planet can not be resolved spatially from the star Combined light of star + planet Modulation of atmospheric signal with time is measured: – At transit we observe absorption features appear and disappear – When wavelengths of planetary features are strongly shifted due to the Doppler effect (orbital motion!) Only signals synchronous with the orbital period can be detected because of all kinds of possible non-planetary signals 50

51 Planck functions 51 Range of temperatures from solar-type stars to transiting planets.

52 Eclipse spectrum 52 Van: http://coolcosmos.ipac.caltech.edu/page/exoplanetshttp://coolcosmos.ipac.caltech.edu/page/exoplanets ‘Contrast’ is the flux from the exoplanet divided by the flux of the star. Usually we know enough of the star to convert that to an absolute flux. The contrast should be as high as possible  observe in the infrared: then the exoplanet emits thermal radiation and the star is much less bright.

53 Eclipse of WASP-12b 53 Measured from the ground at 2 micrometer. Top panel shows the measured data. The middle panels show these same data smoothed. The red curves show a model fit. The bottom panel shows the residuals after subtraction of the model.

54 Phase curve Rotational and orbital periods are the same for hot Jupiters Thus we see the star-facing hemisphere of the exoplanet just before and after eclipse The – colder – other hemisphere is visible near transit When observing an exoplanet over a full orbit we receive radiation emitted from all regions of the exoplanet An observation over a full orbital period is called ‘phase curve’ 54

55 Phase curve of WASP-43b 55 Stevenson et al. obtained this curve in 2017, using the 3.6 and 4.5 micrometer bands of the Spitzer space telescope. The transit is at orbital phase 1.0 and is off-scale on this plot. Secondary eclipses occur at 0.5 and 1.5: intensity left is larger  large difference between day and night.

56 Transits Stellar light that passes through the atmospheric annulus of an exoplanet during transit is affected We measure the transit depth: D(λ) = (F out – F in ) / F out F out is star + planet outside transit, F in is flux during transit D(λ) depends on wavelength when exoplanetary spectral lines are present D(λ) is always positive, although physically the transit is negative 56

57 Transit spectrum of HAT-P-26b 57 The Hubble space telescope has been used in 2017 to derive this transit spectrum showing lines of water, methane and carbon monoxide.

58 Eclipse mapping Solar-type star has an edge as sharp as a knife When an exoplanet is eclipsed by its parent star the brightness distribution of the exoplanet disk can be derived, given enough signal-to-noise One has to invert the profile at ingress and egress (see next Figure) Either reflected light or thermal emission (better!) can be used Measurements have been done with the Spitzer space telescope in the infrared 58

59 Principle of eclipse mapping 59 Hot region on exoplanet is represented by the red spot. Deviations are shown at the bottom.

60 Convolution with spectral templates When individual spectral lines cannot be detected use a set of high-resolution spectra of of star+planet over a wide range of Doppler shifts Determine cross-correlation between template spectrum and observed difference spectra Significant peak in cross-correlation function indicates an match The Dutch astronomer Ignas Snellen first used this technique successfully 60

61 Τau Boötis b 61 Cross- correlation detection of molecular absorption in an exoplanetary atmosphere, carbon monoxide. This planet does not transit!!