投稿類別: 地球科學類 篇名: Life on Exoplanets? 作者: 葉庭妤。台北市立大安高工。綜高二忠 指導老師: 劉宗憲 Life on Exoplanets? I. Foreword 1. Motivation Galaxies, supernovas, extraterrestrials……how starry- eyed I used to be back in my childhood days. Exploring the unknown is extremely mystifying, yet it was only recently that I’ve learned the fact of existing astronomical objects outside the solar system. That’s why I’ve chosen this topic for my research essay. 2. Purpose of Research Many people are desperate to know if any Earth-like planet does appear so that when Earth faces an irrevocable catastrophe there would be a spare region for everyone to immigrate. Furthermore, the potential residents of the target place also prompt us to make more observations of other plausible forms of life and their development in reference to their survival. 3. Methodology As long as the Kepler mission remains active, new discoveries have sprung up like mushrooms each day. The frequently updated database of research essays therefore makes the best information in reach. II. Text Humans have long been pursuing another life-existing planet that is like our own, and ever since the first extrasolar planets (or ‘exoplanets’), the three planets orbiting around a pulsar (a dead star which emits wobbly light that makes it more attractive comparing to other stars) PSR 1257+1 were discovered twenty years ago, people started to have faith in finding candidates. 1. Types of exoplanets We can classify the planets, according to their characteristics, into eight types. Gas giants are the most commonly detected planets, being as big as Jupiter and Saturn, with a hydrogen and helium combination as a body and a metallic core. So are Hot Jupiters, which are only slightly different with gas giants due to its close orbit to its parent star, which made their temperature exceed to about 1300F, and second easier 1 Life on Exoplanets? to be observed. The first exoplanets found were Super-Earths, but the term refers to their larger mass comparing with Earth and possibility to be terrestrial (rocky), not exactly their planetary conditions. Free Floating Planets are meager in detection, and often believed to be ejected from developing systems. Pulsar Planets orbit around Pulsars or Neutron Stars, but were once the core of a star before explosion. Ocean Worlds are considered to be covered with ice and rock initially, but as they move closer to their parent stars, these substances melt into water, and then form oceans. The difference between Cthonian Planets and Super-Earths is often obscure, and the former were once gas giants but drawn too close to their parent stars which leaves them with a rocky surface. Finally, the quest of the mission is Exo Earths. 2. Observation Measures Since planets become many times dimmer as they reflect light from their parent stars, direct imaging is rare most of the time, but there are still exceptions since 2004. Indirect detection methods are crucial in this case, and there are five of them in sum: Pulsar timing, Doppler spectroscopy, astrometry, transit photometry, and microlensing. Fig 1.1 (Pulsar Timing) (A star and planet orbiting around their common centre of mass. http://www.astro.wisc.edu/~townsend/static.php?ref=diploma-2 ) (1) Pulsar Timing Pulsars are neutrons stars that spin in high velocity and are speculated to be the remains of supernovas. They emit radiation periodically, and slight variations of their period can lead to the discovery of those exoplanets even smaller than Earth. However, this detection method is not in scientists’ favor, since pulsars are very rare and the huge stellar explosion nearby can easily omit the chance of life existence, so it was only used once for the first exoplanets ( found by Wolszczan) ever detected. 2 Life on Exoplanets? Fig 1.2 (Doppler spectroscopy) (The principles of the Doppler spectroscopy approach. http://www.astro.wisc.edu/~townsend/static.php?ref=diploma-2) (2) Doppler spectroscopy Doppler spectroscopy is the most successful method in finding exoplanets, which pays attention on the back and forth movement of a star, since gravitational effects are made when two objects are in one system, and by watching the blue- (moving near to Earth) and red (moving far from Earth) shifts of its spectrum, the possibility of an orbiting exoplanet increases. It was first used in 1995 by Meyor and Queloz to discover 51 Pegasi, but as some types of stars wobble because of constant earthquakes, this method isn’t guaranteed to be thoroughly accurate. (3) Astrometry Lalande 21185, discovered by Gatewood, is the only extrasolar system found with the technique of astrometry, the oldest above all methods. Its core is to locate the planet by observing the circular or back and forth movement of its star, and later estimate the mass of the planet. Unfortunately, it is only accessible to those that are near our Earth and due to the angles of observation, the calculation of mass may vary. (4) Transit photometry Fig 1.3(The principles of the transit photometry approach. http://www.astro.wisc.edu/~townsend/static.php?ref=diploma-2) The famous Kepler (Earth-size planets detection) and CoRoT (inner structure of stars probe) missions conducted by NASA and CNES use transit (the parent star, the 3 Life on Exoplanets? exoplanet, and Earth all aligned) photometry to detect exoplanets. This is the most effective way to get information of any planet when elaborate photometers are easy to reach, since the planet in transit is often many times smaller than its parent star, its dimming is often unable to be identified with the naked eye. Unlike the Doppler spectroscopy, this method can measure the density and almost the exact size of the planet, and when the secondary transit occurs (with the star in front of the planet), the temperature map of the planet can be produced by subtracting light of the second transit to the first. But to get a planet confirmed, these two methods must get along. The disadvantages of this technique are that the intervals between transits are usually really long and only those who are viewed edge on can be seen in such activity. (5) Microlensing Microlensing works according to Einstein’s General Theory of Relativity, which predicted that light could be bent by gravitational forces. Imagine that you’re focusing on a star, and another bright object is also behind it, aligned. The ‘background’ object emits bending light which curves beside the lineup, and you can see flares surrounding the star in front of you. When the surrounded light is disturbed, you may presume that there’s an orbiting planet around the target star. This is not a common detection method though. Even though no Exo Earths have been found to date, scientists are confident that the possibility is not close to none. A large portion of extrasolar planetary systems consist of Hot Jupiters, but recent findings unveil those who have the size more similar to our planet. I hope to enter the topic with the analysis of what is required for life to exist. 3. Requirements of our target The basic prerequisites rule out most of the exoplanets we’ve found until now. Many of them are located close to their parent star, which is obviously too hot for general life forms, and also become ‘tidally locked’—the period of their spinning match the period of their stars, causing these planets to orbit with a side always facing their stars and the other the opposite. Eccentric orbital routes are also a problem, since we’re used to the circular movement to our sun which causes the average division of seasons. Having a solid structure is an important concern, too, especially now when we still cannot estimate the composition of a planet. 4 Life on Exoplanets? Fig 2.1(The area of the habitable zone is largest for the massive O, B, and A stars on the upper main sequence. At left, the habitable zone is plotted as a function of spectral type for main sequence stars. Planets inside the "tidal lock radius" rotate once or less per year. http://www.astro.sunysb.edu/fwalter/AST101/habzone.html) The Habitable Zone (or ‘Goldilocks Zone’) is well known to all of those who are enthusiastic about life in extrasolar regions. Sum up the features mentioned in the last paragraph and add another requirement, which is that the star must be fixed to a certain point, points us directly to the idea of the habitable zone. Keep in mind that it is only a calculated distance, according to the mass of the planet, where liquid water may appear. If the planet is too big, the strong gravity may not be suitable for life of our kind, since we won’t be able to take on our weight to move comfortably. The determination of scientists towards Earth-size masses is also due to how the planet grabs its atmosphere. The components of air on Earth are mainly nitrogen and oxygen. When a planet is too large, say, in the size of Jupiter, it will also snatch light gas like hydrogen and helium. When it is too small, only heavier nucleus gases like argon will stay, a planet in this case for instance is Mars. If the atmosphere becomes too thick, the greenhouse effect may disable life preservation. There are also many other examples that prove it’s not enough to stay in this zone. The condition of a planet’s surface is often relevant to its inner structure and mass. A smaller body enables itself to cool down in a short time, and when the inner parts of it are solidified, seismic activity will be decreasing.