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. Gravitational Bending of Light: Microlensing
• The gravitational bending of space causes light to bend its path. • If a foreground star passes in front of a background star, the gravity of the foreground star bends the light of the background star and magnifies its brightness • If the foreground star has a planet, there are two “bumps” in the magnified light – one for the star and one for the planets
30 3. Massive Planets Cause Bigger Magnification
• Massive planets like Jupiter produce bigger microlensing magnification events. • Closer in planets – unlike for radial velocities and transits – are harder to detect • More sensitive to planets in the outer edges of the solar sytems
31 3. Microlensing From Space will Find Earth and Mars like Planets
• NASA is launching the Roman Space Telescope (2026) to perform a microlensing survey • Search for and count the number and frequency of planets in the outer edges of distant solar systems • Sensitivity complements where Kepler left off
32 4. Block the Starlight and Look for the Planet: Direct Imaging
• If you can block the star light without blocking the planet light, you can bring the planets out of the glare • Special instruments called “coronagraphs” are designed to remove the star light and leave behind the planet light
33 4. Larger Planets in Longer Orbits are “Easy”
• GPI on Gemini (US) and Sphere on VLT (EU) are the state of the art coronagraphs • They can detect large gas giant planets that are more distant from their host stars • These instruments can detect planets that are 100,000x to 1,000,000x fainter than their host stars
34 4. Earth-sized Planets Are 10 Billion Times Fainter Than the Sun
• NASA’s next generation space telescopes being designed to improve our abilities to directly image planets • Roman (2026) will have sensitivities to planets that are 10 million to 1 billion times fainter than their host stars • The next NASA flagship mission may be LUVOIR or HabEx (2030s) and are designed specifically to directly detect an earth-mass planet in an earth-like orbit • May use an external starshade to block the star light
35 Exoplanet Atmospheric Characterization with the Spitzer and Hubble Space Telescopes and Future Prospects with JWST
?
Dr. Nikole Lewis Assistant Professor, Cornell University Deputy Director, Carl Sagan Institute What is their air like?
36 One of the primary ways that Spitzer, Hubble, and Webb space telescopes will probe exoplanets is via the transit method
37 The chemical fingerprints of exoplanet atmospheres
Wavelength PlanetaryRadius
38 Helium Carbon Monoxide
Methane Titanium Oxide
Clouds
Hazes
Water Potassium Sodium
39 Hubble and Spitzer have provided the first atmospheric probes of habitable zone worlds
TRAPPIST-1 TRAPPIST-1 K2K2-18b-18b
40 Beyond Transits: Probing Direct Emission from Exoplanet Atmospheres
41 Hubble and Spitzer have given us a glimpse at the weather and climates of distant worlds
42 In the era of Webb Potential for >300 Exoplanets with Characterization Observations
In the era of Hubble & Spitzer >100 Exoplanets with
Characterization Observations 43 Webb will provide an important new window into exoplanet atmospheres
44 Webb will provide compositional information for a large sample of giant exoplanets
Estimated JWST Precision
Bean et al. (2018)
Bean et al. (2018)
Shabram et al. (2011)
45 Webb will help to determine the natureECLIPTIC POLE of 24°$ Jr Super-Earths and Mini-Neptuneseclip&c' pole'
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Figure 7. Left.—The instantaneous combined field of view of the four TESS cameras. Middle.—Division of the celestial sphere into 26 observation sectors (13 per hemisphere). Right.—Duration of observations on the celestial sphere, taking into account the overlap between sectors. The dashed black circle enclosing the ecliptic pole shows the region which JWST will be able to observe at any time.
the 1.5 N m of angular momentum build-up induced by solar radiation pressure. For this purpose TESS uses its hydrazine⇡ thrusters.
7.4 Ground-based data analysis and follow-up The TESS data will be processed with a data reduction pipeline based on software that was developed for the Kepler mission.22 This includes pixel-level calibration, background subtraction, aperture photometry, detrending with respect to weighted ensembles of target star light curves, and searching for transits with a wavelet-domain Transiting Exoplanet Surveymatched filter. Satellite (TESS) Ricker et al. (2014) Once the data are processed and transits are identified, selected stars will be characterized with ground- based imaging and spectroscopy. These observations are used to establish reliable stellar parameters,Greene et al. confirm (2016) the existence of planets, and establish the sizes and masses of the planets. Observations will be performed with committed time on the Las Cumbres Observatory Global Telescope Network and the MEarth observatory. In46 addition the TESS science team members have access to numerous other facilities (e.g., Keck, Magellan, Subaru, HARPS, HARPS-North, Automated Planet Finder) through the usual telescope time allocation processes at their home institutions. The TESS team includes a large group of collaborators for follow-up observations and welcomes additional participation.
8. ANTICIPATED RESULTS 8.1 Photometric performance Figure 8 shows the anticipated photometric performance of the TESS cameras. The noise sources in this model are photon-counting noise from the star and the background (zodiacal light and faint unresolved stars), dark current (negligible), readout noise, and a term representing additional systematic errors that cannot be corrected by detrending. The most important systematic error is expected to be due to random pointing variations (“spacecraft jitter”). Because of the non-uniform quantum e ciency of the CCD pixels, motion of the star image on the CCD will introduce changes in the measured brightness, as the weighting of the image PSF changes, and as parts of the image PSF enter and exit the summed array of pixels.