2. Extrasolar planets and planetary systems Extrasolar database: http://exoplanet.eu as of March 30, 2020
• 4236 confirmed exoplanets from all techniques in 3136 planetary systems, of which 691 are multiple (>1 planet). • 882 from radial velocity in 650 systems, of which 156 are multiple. • 3031 from transit in 2272 systems, of which 494 multiple. • 136 from direct imaging in 104 systems of which 4 are multiple. • 114 from micro-lensing in 105 systems of which 5 multiple. • 10 from astrometry. • Plus about 2500 candidates nearly all from Kepler transits
Note: most of the exoplanets have been found by transits from the NASA Kepler mission, which “stared” at all stars in a single 115 deg2 area (0.3% of the sky). The host stars of these planets are therefore faint and distant, limiting follow-up possibilities. There are also many Kepler candidates which were not seen to repeat. Planetary mass vs. year of discovery
Jupiter mass
Earth mass
1321/4236 with known mass • Planets detected down to Earth masses • Multiple systems (up to n = 6) • Planets around binary stars (both around individual stars and both) • Even multiple systems around binaries!
But, severe selection effects are obvious: • Radial velocities: planetary mass, distance from host star, period, and the spectral suitability of star • Transit: planetary size, distance from host star, size of host star, period • Direct imaging: size, mass, angular distance from host star, contrast with star • Micro-lensing: not so many selection effects, but very rare • Astrometry: mass, distance from host, period, distance from us
N.B. Kepler candidates are generally very distant (small faint survey field) Surprise: The first planets found using RV (1995) “Hot were “hot Jupiters” – Jupiters” Jupiter-mass but very close to the star. Jupiter Easiest to detect but quite unlike Solar System and our understanding of planet Earth formation.
Cannot form there: Likely migrated in from further out. Many planets may have migrated all the way in to the star?
We are just now able to detect Earth-like mass at Earth-like radius around Solar- type stars (e.g. Kepler 452b found in 2015) Most planets are orbiting co-planar with the star’s rotation axis (see Rossiter-McLaughlin method last lecture). Orbit well-aligned with stellar spin Some (almost all hot Jupiters) are not, and a Orbit perpendicular to spin few are even counter orbiting. Orbit opposite to stellar spin Invalidate formation ideas? No, generally thought to reflect the Orbit perpendicular to spin Lidov-Kozai mechanism, a complex three-body Orbit well-aligned with stellar spin interaction of a distant third body with a binary.
o o o Also, note planets many Note: angles wrap at 360 , so 330 = -30 have substantial orbital eccentricities e >> 0 See later Most host stars are similar to the Sun. But note that there are multiple selection effects operating here: Choice of (quiescent) stars for spectroscopy. Size of star for transit (size of effect and likelihood of transit in opposite sense) From review by Adibekyan 2019 Early on, it was realized that stars hosting planets are generally metal-rich relative to the stars without planets. Again, there are multiple SUN selection effects operating here, but these are relatively easy to take out.
This is well established for stars hosting Jupiter- mass planets. Original ideas: • Formation? The mass of heavy elements in the planet-forming disk affects grain growth and masses of planets [This is now the generally accepted explanation] • Consumption? Metals in stellar atmospheres due to pollution from the consumption of migrating planets. Only low mass With low mass Note: Our Sun has a metallicity near the lower planets planets end of such Jupiter-hosting systems.
But, there is some evidence that stars that host only low mass planets (no Jupiters) have a lower metallicity distribution (the same metallicity distribution as stars apparently without planets). Probably the most surprising result: Most exoplanets are on much more eccentric orbits (from RV) compared with the planets in the Solar System. • No obvious selection effects favouring detection of high e. • e-distribution more like that of binary stars? Linked to scattering?
Almost certainly linked to the multiplicity of the planets:
• Origin uncertain (migration of Jupiter??). • Eccentric planets might not survive in high-n systems (orbit overlap) • May suggest that most planets on eccentric orbits will not be found to have Limbach & Turner 2015 many companions. Bottom line: how common are planets around Solar-type stars?
Reasonable estimates: • About 1% have a “hot Jupiter” • About 6% have Jupiter within 5AU Multiply by at least x2 for known selection effects.
Bottom line, it is reasonable to assume (order of magnitude) • Average of about one planet per star • About 20% of Sun-like stars have a more or less “Earth-like” planet on a more or less “Earth-like” orbit
Conclusion: planets are common in the Universe Densities can be determined from the combination of sizes from transits and masses from radial velocities. Furthermore, if you see a transit, you know the inclination i ~ 0, removing the sin i uncertainty.
From review by Adibekyan 2019
gas giants
rocky
Structure of exoplanets very similar to in Solar System. Note the existence of rocky planets up to ~ 10 Mearth (“superearths”). Kepler transit searches have been very productive
Kepler 20e and 20f in 5 planet system (periods of 6 and 20 days) Estimated 0.4-1.7 and 0.66-3.0 Mearth Alpha Centauri B (Proxima Centauri), is the closest star to the Solar System at a distance of 1.3 pc (4 ly).
In 2016 (Anglada-Escudé et al): A roughly Earth- mass (1.3 to 8 Mearth) planet orbiting 0.05 A.U. (P=11.2 days) from a 0.12 Msun red dwarf star, found by radial velocity of 1.4m/s. No transits. Tsurface ~ 230K !!
A visit is not inconceivable….. 3. The search for “habitable worlds” Conditions and consequences for Life on other planets?
What conditions would we think to be optimal? • A surface with the physical conditions allowing liquids (esp. H2O). • Pre-biotic materials (C-based organics etc) We have seen that these are a reasonable outcome from planetary formation.
Could we hope to detect such a world as seen from Earth? Remotely-detectable signatures would include: • Possibly altered atmospheric chemistry (esp. out of equilibrium). • Possible surface spectroscopic signatures seen through a transparent
Title atmosphere. Life in the Universe © Simon Lilly 2002 The “Habitable Zone” – traditionally defined as the distance from star at which the usual surface equilibrium temperature (in absence of greenhouse effects, and assuming typical albedo) allows liquid water. Note that Life may well exist outside of this, e.g. Jovian moons.
1. Habitable Zone will migrate as luminosity of star changes (e.g. we expect the luminosity of the Sun to have increased by +30% over last 4 billion years) ®idea of “Continuously Habitable Zone” 2. Location of the Habitable Zone depends on the mass (i.e. luminosity/temperature) of the star. Note that we can also compute the region where planets should be tidally locked (includes entire HZ for low mass stars) Note that the actual surface temperature depends on:
1. Distance from Sun/Star, D, and the properties of the Sun/Star 2. Albedo (fraction of solar energy absorbed), A 3. Day-Night variations – depends on the density of the atmosphere, tidal locking of spins with orbits, day-night DTD-N driving winds etc…. 4. Greenhouse effects in the atmosphere, DT
(1) can be measured easily (2) could be inferred if planetary size is known (e.g. transit) (3) & (4) quite hard to measure or predict, especially if there are high altitude clouds obscuring the lower atmosphere. So, what could we observe looking at the Earth from far away?
The Earth is actually well-suited to being remotely observed! • Surface reflectance features seen through a transparent atmosphere (at visible wavelengths). • Atmospheric composition diagnostics
Title Life in the Universe © Simon Lilly 2002 Visible/near-IR reflectance spectra of portions of Earth’s surface
Chlorophyll Reflectance spectrum of green leaf Integrated spectrum with Individual components 70% ocean and 40% clouds
desert
forest ocean
Observed spectrum of Earth (Earthshine from the dark side of moon) Rotation signature of continents and oceans during the day as measured at different wavelengths
but (moving) clouds will have randomizing effect… Variability of the “red edge” signature (model spectrum with actual clouds) Absorption in planetary atmospheres seen against the thermal emission of the surface Comparison of Venus, Earth, Mars in mid-infrared
SO2 Venus CO2
CO2
Strong sharp ozone absorption at 9 µm CH Earth 4 H2O O3
Mars Note: the detection of both reducing and oxidizing gases (e.g. CH4 and O2/O3) CO2 CO2 indicates atmosphere far from chemical equilibrium, being pumped by a source of one or more likely both But note that the modified thermal emission of Earth due to its atmosphere depends strongly on the altitude of cloud cover. If the clouds are above most of the atmosphere, you don’t see much at all. EXOGENIC PROCESSES STELLAR RADIATION
RADIATIVE TRANSFER
CLIMATE ATMOSPHERIC CHEMISTRY How difficult it will be to make
T(z)reliable a priori predictions about anything
WEATHERING
Compexity of planetary properties Simulated Venus, Earth, Mars at a distance of 10 pc
Simulated Earth spectrum
2007 Proposed ESA Darwin mission (no longer in active development due DARWIN/TPF etc to cost) 4. The search for “inhabited worlds” The Search for Extraterrestrial Intelligence (SETI)
• How many “civilisations” exist? • How eager are they to talk? • How could we communicate?
Three questions How many? The Drake equation
The number of “civilizations” in (our or any other) galaxy at any given time is given by: Number of stars in the Galaxy 3 ´ 1011 ´ ´ Fraction of suitable stars 0.1 ´ ´ Fraction with planetary systems 0.1 ´ ´ Number of suitable planets per system 1? ´ ´ Fraction that actually produce Life 0.1 ?? ´ ´ Fraction that develop “civilization” 10-3 ??? ´ ´ Ratio of the lifetime L of that civilisation L / 1010 Express lifetime to the age of the Milky Way (~10 Gyr) as “L” simply because of the huge My own best estimate: N ~ L / 30,000 uncertainty Various estimates….. NSJL ~ L / 30,000
Even the factors before the lifetime L factor are uncertain at the 10±6 level, so we can’t say much. But two statements can be made: • It is quite possible that we are acually “alone” in the Galaxy (= no other “civilization” at the present time). • But if we multiply by 1011 galaxies, then even the worst case has N ~ 10 L for the whole Universe (we may be confident L >> 1). So, it is unlikely that we are completely alone in the Universe. How far away is the nearest (living) civilization today? NSJL ~ L / 30,000
0.3-0.5 Distance to our nearest neighbor in the Galaxy dcivil ~ (N* / Ncivil) d* N* ~ number of stars in the galaxy d* ~ typical distance between nearest stars ~ few light years Exponent depends on whether system is 2-d or 3-d (which depends on N).
Note that Ncivil ~ 100 already implies distances (light travel times) of order a few thousand light years.
This plot then shows Ncivil as a function of the lifetime L for any choice of the non-L factors in the Drake equation. This then sets dcivil as above. We can then see how many two- way messages (at speed of light) that we could send this distance in the available time L.
Conclusion: A meaningful my best N=L / 30,000 conversation will be difficult. Will civilizations have a desire to communicate?
In favour: Against: Curiosity Fear Sociability Ignorance Avarice Other priorities
What about eavesdropping on non-communicative (or even long-dead) civilisations? We are broadcasting our presence….
Eavesdropping: radars and entertainment emission** ** temporary phases?
Title Where to look for intentional signals? The so-called radio “water-hole” at 2 GHz
~ 2 GHz the so-called “water-hole” is at the minimum of Ideally need to search 2 GHz at the background emission in Milky Way. Called “water- roughly 1 Hz resolution (to hole” because it is between frequencies of prominent H detect m/s Doppler shifts), i.e. and OH transitions (and because civilizations might need to cover ~ 2 billion congregate there!) spectral channels at once. Allen Telescope Array (ATA) Upto 350 ´ 6-m modified satellite TV dishes (104 m2 area). The plan was to
monitor ~ 100,000 stars (so need Ncivil > 30,000 for success) But, with my “best guess” Drake value, this requires L ~ 109 yr. Unlikely? In my view, SETI at this level is a “long shot”, but what better way is there to spend your money!
Allen telescope Exosolar planets: key points Over 4000 exo-planets around other stars have been detected since 1995, both (most) indirectly and directly. But all these methods still have substantial selection biasses. Especially in combination, one can learn quite a lot: masses, sizes, densities, atmospheric composition (+ albedo, spin, etc) Planets, similar to those in the Solar System are common in (our) Galaxy. A few Earth-like planets around Solar-like stars on more or less Earth-like orbits have been found. Some exo-planetary systems show some surprises, likely linked to dynamical processes towards the end of planetary system formation. • Hot Jupiters close to the parent star show migration was important. • The Solar System’s low eccentricities are unusual and likely linked to it’s high multiplicity. Remote detection of bio-signatures is feasible – the Earth would be especially detectable: e.g. Chlorophyll and O2+CH4 are quite detectable. We may well be the only “civilization” in the Galaxy “at the present time”, but it is unlikely we are alone in the Universe. Meaningful communication will be difficult.