Science Briefing March 11, 2021 Exploring Exoplanets Today and Tomorrow Dr. Courtney Dressing (University of California at Berkeley) Dr. David Ciardi (NASA Exoplanet Institute/IPAC/Caltech) Dr. Nikole Lewis (Cornell University) Facilitator: Dr. Quyen Hart (STScI) Outline of this Science Briefing 1. Dr. Courtney Dressing (University of California at Berkeley) Finding Planets Orbiting Nearby Small Stars 2. Dr. David Ciardi (NASA Exoplanet Institute/IPAC/Caltech) Why Is It So Hard to Find Earth-sized Planets? 3. Dr. Nikole Lewis (Cornell University) Exoplanet Atmospheric Characterization with the Spitzer and Hubble Space Telescopes and Future Prospects with JWST 4. Q&A 5. Dr. Quyen Hart (STScI) NASA Educational Resources 6. Q & A 2 Finding Planets Orbiting Nearby Small Stars Dr. Courtney Dressing Assistant Professor of Astronomy Credit: NASA/JPL-Caltech/MSSS University of California, Berkeley 3 Our Solar System Is Home to Eight Planets • How do astronomers find planets? • How common are planetary systems orbiting other stars? • Do other planetary systems look like our own? Illustration not to scale 4 Astronomers Use Multiple Techniques to Find Planets Radial Velocity Observations Transit Observations Reveal Planet Masses Reveal Planet Sizes 5 The Doppler Effect Changes the Apparent Pitch of Moving Sirens • When the truck is stationary, all listeners hear the siren at the frequency at which the sound is emitted from the firetruck. • When the truck is moving, the frequency of the sound waves depends on the direction of motion. • The frequency is higher when the truck is moving towards the listener. • The frequency is lower when the truck is moving away from the listener. Image Credits: NASA’s Imagine the Universe 6 The Doppler Effect Also Changes the Color of Starlight • Planets tug on stars as they orbit, which causes the star to move. • Just like the sound of an ambulance changes pitch as it drives by, the color of starlight changes based on the motion of the star. • Starlight looks bluer when stars are moving towards us. • Starlight looks redder when stars are moving away from us. Image Credits: NASA’s Imagine the Universe 7 The Radial Velocity (Doppler Wobble) Method Reveals Planet Masses • Astronomers can measure the color change to determine the properties of the planet. • Planets that are closer to their stars orbit more quickly, so the light changes color more quickly. • More massive planets cause more extreme shifts in starlight color. Image Credit: Las Cumbres Observatory 8 In 2012, Venus Transited (Crossed In Front Of) the Sun Image Credit: NASA 9 The Transit Method Reveals Planet Sizes • When planets cross in front of stars, they temporarily block some of the light from the star’s surface. • Astronomers can monitor the brightness of a star versus time to determine the properties of the planet. • Planets that are closer to their stars orbit more quickly, so the transit events are shorter and occur more frequently. • Larger planets block more light than smaller planets. Image Credit: Hans Deeg 10 Most Planets In the Solar System Would Be Difficult To Detect Image Credits: NASA Signal Jupiter Orbiting the Sun Earth Orbiting the Sun Radial Velocity 12.5 m/s 0.09 m/s = 9 cm/s Transit Depth 1% 0.0084% = 84 ppm 11 Most Stars In The Galaxy Are Smaller Than The Sun • The solar “neighborhood” contains hundreds of stars. • Some stars are larger, hotter, and more massive than the Sun. • Most stars are smaller, cooler, and less massive than the Sun. • Planets need to be closer to smaller & cooler stars to have the same surface temperature. Image Credit: Todd Henry/RECONS 12 Radial Velocity Observations Revealed A Potentially Low-Mass Planet Orbiting Our Nearest Neighbor Star Mass = 12% MSun 4 light years away Planet Mass ≥ 1.27 Mearth Period = 11.186 days Habitable Zone periods: 9-25 days Anglada-Escude+ 2016, Nature, 536, 437 13 Transiting Planets Orbiting Smaller Stars Are Easier To Detect Sun-like Star Red Dwarf • The transit depth is proportional to the area of the planet divided by the area of the star • Increasing the planet size and/or decreasing the star size leads to deeper, more detectable transits. Image Credit: Planet Hunters 14 Planetary Transits Are Rare And Challenging To Detect • Planets must lie between the observer and the star to be seen in transit, which means that most planets never transit. • Some planets take a very long time to orbit their stars and transit infrequently. • Planets that do transit spend only a small fraction of their orbits in front of their host stars. • Some planets are so small that they do not block enough stellar light for their transits to be detectable. www.planethunters.org NASA SDO 15 How Can Astronomers Maximize the Odds of Finding Transiting Planets? • There are two primary approaches: 1. Concentrate on a small number of stars with a high likelihood of hosting transiting planets. 2. Observe many stars simultaneously. NASA/Carter Roberts/Eastbay Astronomical Society NASA 16 Space-Based Surveys Can Find Smaller, More Distant Planets • Although any given star has a low Image Credit: Zach Berta-Thompson likelihood of being transited by a planet at any particular instant, monitoring the brightness of many stars for an extended period of time can reveal many planets. • NASA’s Kepler and K2 missions monitored hundreds of thousands of stars and revealed thousands of planet candidates. • NASA’s TESS mission is continuing Kepler’s legacy by monitoring the brightness of bright, nearby stars 17 TESS Has Already Discovered Thousands of Planet Candidates Credit: NASA/MIT/TESS 18 TRAPPIST-1: A Multi-Planet System Orbiting A Very Small Star Credit: NASA/JPL/Caltech 19 Summary • Astronomers are now able to detect small and low-mass planets. • Large, space-based surveys have detected thousands of transiting planets. • Ground-based surveys have detected particularly interesting planets orbiting the nearest stars. • The cool temperatures, low masses, and small radii of Red Dwarf stars provides a fast-track route to detecting smaller, cooler, and possibly even Earth-like planets. • Upcoming JWST observations of transiting planets will improve our understanding of planetary atmospheres. Credit: NASA/JPL-Caltech 20 Why Is It so Hard to Find Earth-sized Planets? • Humanity is on a quest to find planets around other stars that may resemble the Earth – and perhaps host life of their own. • This talk is about Dr. David Ciardi the extreme challenges involved in finding and characterizing those planets. 21 The Sun is Bigger Than Jupiter … and Way Bigger Than Earth • The Sun is 10 times bigger than Jupiter and 1000 times more massive than Jupiter • The Sun is 100 times bigger than Earth and 330,000 times more massive than Earth Four Methods to Find the Planets Around Other Stars 1. Measure the wobble of the star as the planet orbits the star (radial velocity) 2. Measure the brightness of the star as the planet crosses in front of the star (transits) 3. Measure the increase in brightness of a background star as a planet passes in front (microlensing) 4. Block out the starlight and search for the light from the planets (direct imaging) 22 1. Measuring the Star’s Wobble: Radial Velocity Method • Spectroscopy of stars splits the light of the stars into its various wavelengths – dark lines in the spectra caused by different elements in the star atmosphere (e.g., H, Na) • As the planets orbit their stars, the gravitational tuck on the stars makes the stars wobble • That wobble makes the star move toward and away from our view – causing the apparent wavelength of the atmosphere lines to shift back and forth 23 1. Massive and Short-Period Planets Are “Easy” to Detect • Massive planets (e.g., Jupiter) and short-period (e.g., days) induce a large radial velocity shift on the stars • Earliest planets found were Jupiter-mass, in orbital periods of days à radial velocity shifts of 10’s to 100’s m/s • Jupiter orbits the Sun in 12 yrs producing a radial velocity shift on the Sun of about 12 m/s 24 1. Earth-mass Planets in Earth-like Orbits Have Low RV Signatures • Earth orbits the Sun in 1 year producing a radial velocity shift on the Sun of about 9 cm/s • The Sun is a giant ball of gas that pulsates producing radial velocity shifts 20× larger than the Earth signature (2 m/s vs 9 cm/s) • The next decade will see a revolution in instrument precision and techniques to isolate the stellar noise from the planet signal • NASA investing in the development of the newest instruments 25 2. Measuring the Brightness Dip: Transit Method • If a planetary system appears edge-on to our line-of-sight, the planets may pass in front of the star and block star light once per orbit (i.e., the planet transits) • If we monitor the brightness of thousands of stars, we can get lucky and find planets as they transit in front of their host stars. 26 2. Large and Short Period Planets Are “Easy” to Detect • Large planets like Jupiter produce transits that are 1% or deeper • Planets repeat the transit in every orbit making short period orbits easier to detect than long period orbits 27 2. Earth-sized Planets in Earth-like Orbits Have Small Transits • Earth-sized planets produce transits that are 100x smaller • And if in 1-year orbit, need to monitor the stars for at least 4 years to ensure 3 transits are measured 28 2. Earth-sized Planets in Earth-like Orbits Have Small Transits • NASA launched Kepler (2009-2018) to search for Earth-sized planets in Earth- like orbits • Kepler-452b is best candidate • 1.6x Earth-size • 384 day period • ESA launching transit mission PLATO (2026) to search for Earth-sized planets in Earth-like orbits 29 3.