Lab10- PHYS1403.901

Circumstellar Habitable Zones/Extrasolar Planets Image of the Kepler Space Telescope's Search Range inside this arm of the Milky Way Image Courtesy of: www.dailygalaxy.com The Milky Way Galaxy is populated with billions of stars like our Sun, and long before we had the scientific capability to prove the existence of other planets around those stars, we dreamed of those foreign worlds. With no evidence outside our own solar system, we dreamed of the extraterrestrial terrains explorers might encounter. Science Fiction and fantasy is filled with fantastic fictional encounters with alien civilizations on those worlds. The idea of extrasolar planetary travel intrigues the human inclination to explore. Sadly, technology has not caught up to science fiction with respect to space exploration, but it has achieved positive proof of the existence of other worlds. 1

Artist's Impression of the Planets around Pulsar, PSR B1257+12. The first confirmed extrasolar planets. Image By: NASA/JPL-Caltech/R. Hurt (SSC) The first extra solar planet was actually discovered in 1992, when several planets were detected orbiting a pulsar. The first extrasolar planet orbiting a main sequence star came three years later, and is from a class that came to be referred to as a 'hot jupiter' planet, being so close to its star that it may reach as high as a roaring 1200 Kelvin on the day side. Since those first discoveries though, astronomers have found thousands of extrasolar planets, largely by the work of the Kepler Spacecraft and team. These discoveries range in mass from approximately Earth sized to nearly star sized (these objects are called brown dwarfs). Since that discovery, we've also established several other methods for Extrasolar planet detection that have broadened the search parameters to include Earth-scale planets. Current statistical estimates made by extrapolating from the density of extrasolar planets in nearby star regions, indicate that each star in the Milky Way should have at least one planet orbiting it. In January 2013, the Harvard-Smithsonian Center for Astrophysics announced the expectation of at least 17 billion Earth sized planets in the Milky Way alone. The key to detecting these planets is to employ a variety of search methods over widely different parts of the galaxy, because each method of extrasolar planetary detection has its' own strengths and weaknesses. 2

may not immediately realize is that just as you are being pulled down to the Earth, the Earth is being pulled towards you. The reason this reaction on the Earth isn't immediately noticeable is the Earth has a mass of 5.97x10 24 kg, and you might have a mass of 100 kg. So you could never measure the tiny acceleration your gravity causes on the Earth. The same feature applies to stellar gravity. As the Sun pulls on the Earth and other planets to keep them in orbit, the planets pull on the Sun. But in this case, the combined masses of the planets is actually enough to make a measurable force on the Sun. Saying that the planets orbit the Sun is actually a simplification. Really, the Sun and planets both orbit their combined center of mass, in a sort of massively complex dance. That center of mass is still inside the Sun's radius, for our Solar system, but this means that an outside observer would see that the Sun wobbles around while the planets orbit! Therefore, if scientists want to find other stellar systems, they can watch for the star to wobble as it dances gravitationally with its planets! Graphic of Radial Velocity Method Image by: http://www.scienceinschool.org/2011/issue19/exoplanet Detecting the wobble of the star becomes a matter of looking for the Doppler Effect in its light spectrum. When a light source is moving radially away from us, the light appears to have a longer wavelength, this is known as red- shifting because red has the longer wavelengths in the visible spectrum. As the light source moves towards us radially, its light appears to have shorter wavelengths, known as blue-shifting. So by watching the spectrum of the star as it dances with its planets, we can see the whole spectrum shift towards the red when it's moving away, and to the blue when it's moving towards us. Circumstellar Habitable Zone 4

Image by:Planetary Habitablility Laboratory at University of Puerto Rico, at Arecibo. http://phl.upr.edu/ In the field of extrasolar planetary hunting, it is an exciting achievement to discover a planet of any description. The true treasure that every team is hunting for though, is another planet that can support life as we know it. As far as biologists have been able to tell, there is only one feature of the Earth that every one of its organisms requires for survival: the presence of liquid water at its' surface. Thus the hunt is on for extrasolar planets with conditions that allow liquid water at their surface. Such a planet is in the Circumstellar Habitable Zone (CHZ), or “Goldilock's Zone” around its star. It's far enough from its star so that water wouldn't boil away (not too hot), close enough that water wouldn't freeze to ice at its surface (not too cold), and the right size to allow molecular hydrogen to be trapped on its surface by gravity (just right). There are a lot of complications to this idea. Firstly, that the CHZ around a star would change dramatically as the star ages in its life cycle, changing luminosity and size. Secondly, that size and composition of atmosphere for a planet can dramatically change the temperature at its surface. However, as a means of ruling out planets where life as we know it could not possible exist, the concept of the CHZ is extremely useful. 5

3) Calculate the Habitable Zone inner and outer boundaries for each chosen star. This is done using the following equations. Record your results in the table and again, show your work for credit: 4) Now look back at the Semi-Major Axes for each star that you recorded from NASA's database. Which of the exrasolar planets in each system fall in the Circumstellar Habitable Zone for their star? Hint: Identify which stars have a semi-major axis that lies between R inner and R outer . Note: We are ignoring the fact that several of these planets have highly elliptical orbits. Although the semimajor axis may (or may not) lie in the CHZ, a large eccentricity may take the planet in and out of the CHZ during a full orbit. Part 2 5) The Sun We will use the information available to determine which, if any, of the planets in our own Solar System are in the habitable zone of the Sun. Use the data in Table 2 and find the inner and outer boundaries for the habitable zone for the Sun. 6) Determine whether Venus, Earth, and Mars are in the Sun’s habitable zone using this method and record your data in the answer sheet. 7

Table 1: Extrasolar Planet Data Star Name Planet Letter Discovery Method Orbital Period (days) Stellar Distance (pc) Semi- Major Axis (AU) Eccentricity Planet Mass (Jupiter Mass) Stellar Mass Spectral Type Stellar Luminosity (logL solar ) 55 Cnc f Radial Velocity 262 12.53 0.788 0.305 0.141 0.91 G8 V -0.235 61 Vir c Radial Velocity 38.021 8.52 0.218 0.14 0.057 0.94 G5 V -0.095 BD+14 4559 b Radial Velocity 268.94 50.03 0.777 0.29 1.47 0.86 K2 V -0.32 BD+48 738 b Radial Velocity 392.6 1 0.2 0.91 0.74 K0 III 1.69 BD-17 63 b Radial Velocity 655.6 37.54 1.34 0.54 5.1 0.74 K5 V -0.678 GJ 667 C c Radial Velocity 28.14 6.8 0.125 0.02 0.012 0.33 M1.5 V -1.863 GJ 667 C f Radial Velocity 39.026 6.8 0.156 0.03 0.008 0.33 M1.5 V -1.863 GJ 687 b Radial Velocity 38.14 4.53 0.164 0.04 0.058 0.41 M3 V -1.672 GJ 832 c Radial Velocity 35.68 4.94 0.163 0.18 0.017 0.45 M1.5 -1.585 GJ 876 c Radial Velocity 30.088 1 4.7 0.13 0.25591 0.7142 0.33 M4 -1.886 GSC 06214- 00210 b Imaging 145 320 16 0.9 K7 -0.42 HD 100777 b Radial Velocity 383.7 52.8 1.03 0.36 1.16 1.01 G8 V 0.021 HD 125612 b Radial Velocity 559.4 52.83 1.37 0.46 3 1.09 G3 V 0.037 HD 137388 A b Radial Velocity 330 38.45 0.89 0.36 0.223 0.86 K2 IV -0.337 HD 13908 c Radial Velocity 931 66.89 2.03 0.12 5.13 1.29 F8 V 0.602 HD 141937 b Radial Velocity 653.22 33.46 1.488 0.41 9.316 1.03 G2/3 V 0.022 HD 142415 b Radial Velocity 386.3 34.57 1.05 0.5 1.62 1.03 G1 V 0.06 HD 147513 b Radial Velocity 528.4 12.87 1.32 0.26 1.21 1.11 G3/5 V -0.01 HD 1502 b Radial Velocity 431.8 167.5 1.31 0.101 3.1 1.61 K0 1.064 8

Table 2" Orbital Data for the Solar System Inner Planets Planet Eccentricity Semi-Major Axis (AU) Stellar Luminosity (logL solar ) Venus 0.0 0.7 0.0 Earth 0.0 1.0 0.0 Mars 0.0 1.52 0.0 10

Answer Sheet Selected Extra-Solar Planets Star Name Planet Letter L Star (Solar) Semi- Major Axis (AU) R inner (AU) R outer (AU) In Habitable Zone? Yes/No 1 BD-17 63 b 10 -0.678 = 0.21 1.34 √(0.21/1.1)= 0.44 √(0.21/.53)= 0.63 NO 2 GSC 06214- 00210 b 10 -0.42 = 0.38 320 √(0.38/1.1)= 0.588 √(0.38/.53)= 0.847 NO 3 HD 100777 b 10 0.021 = 1.05 1.03 √(1.05/1.1)= 0.977 √(1.05/.53)= 1.41 YES 4 HD 147513 b 10 -0.01 = 0.98 1.32 √(0.98/1.1)= 0.944 √(0.98/.53)= 1.36 YES 5 HD 218566 b 10 -0.452 = 0.35 0.688 √(0.35/1.1)= 0.56 √(0.35/.53)= 0.813 NO 6 HD 40307 b 10 -0.639 = 0.23 0.6 √(0.23/1.1)= 0.457 √(0.23/.53)= 0.659 NO 7 HD 4113 b 10 0.086 =1.22 1.28 √(1.22/1.1)= 1.053 √(1.22/.53)= 1.52 YES 8 HD 44219 g 10 0.26 = 1.82 1.19 √(1.82/1.1)= 1.286 √(1.82/.53)= 1.85 NO 9 Kepler- 443 b 10 -0.664 = 0.22 0.495 √(0.22/1.1)= 0.447 √(0.22/.53)= 0.64 YES 10 Kepler-47 c 10 -0.076 = 0.84 0.991 √(0.84/1.1)= 0.874 √(0.84/.53)= 1.259 YES 11