篇名:Life on Exoplanets?

Home , Doppler spectroscopy, Gliese 581g

投稿類別: 地球科學類

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

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.

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

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.

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. If an atmosphere exists on the planet, rainfall may be produced, and later form oceans and lands. It is also suggested that the formation of life in another solar system should require a base atom (as in ‘carbon’) to generate complex life forms, a master molecule (as in ‘DNA’) to duplicate itself, and a solvent (as in ‘water’) to join these atoms and molecules together. The Urey-Miller experiment, which simulates Earth’s primitive atmosphere, also concluded that with strikes of lightning, amino acids could be formed, too. Although these mentioned above are not life themselves, they are what pushed the first of lives.

Interference to a planet can be as vital, even though most people forget the

Life on Exoplanets? jeopardy that astral substances may pose to us when they are busy evaluating other statistics about it. Sometimes a super explosion may affect the planet even if it’s in the habitable zone. To apply an extrasolar system with the model of our own, planets high in mass like Jupiter near an asteroid belt or moons low in mass near the target planet could be best to avoid asteroids and fragments to damage the planet.

Stars are important to planets to some extent. It’s recommended that there shouldn’t be binary stars nearby the target planet, since its orbit might be unstable and frequent collisions may occur, though scientists believe within a few more researches they will find a system of such to be perfectly balanced. Stars of type F (blue to white),G (white to yellow),K (orange to red), and M (red) have longer life spans than others, which also encourages the chance of life existence . Meanwhile, we have to notice the energy that our sun had emitted has increased 30% since the formation of Earth.

4. Candidates

As the Kepler mission goes on, five candidates have been listed as potential habitable planets until now (ranked with likeliness respectively, the ratio of similarity in parenthesis): Gliese 581g (0.92), Gliese 667Cc (0.85), Kepler-22b (0.81), HD85512b (0.77), and Gliese 581d (0.72). Those with evaluations more than 0.8 are more likely to be Earth-like, while others that are about 0.7 might only have life in microbial forms. Gliese 581g is regarded as the most promising of the five, and its existence had been suspected when scientists found Gliese 581d. All of the candidates are Super-Earths, which means that they are about 1 to 10 times the mass of Earth’s. Here I have to insert an important piece of information, so that you can get the background of potential habitable exoplanets:

In 2012, ESO’s HARPS planet finder observed that about 40% of all red dwarf stars have a Super-Earth orbiting in the habitable zone. Moreover, more massive planets tend to be rare around them. There are about 160 billion of red dwarfs in the Milky Way, which makes them very common in the universe, and gaining this fact may shed light to Exo-Earths. Unfortunately, red dwarfs also make the planets around exposed in X-rays or ultraviolet radiation. Even so, astronomists are still eager to learn about the atmosphere of the planets and life signs at the same time. (personal adaption of Hook, Richard. 2012.)

The listed exoplanets with their names started with Gliese orbit around red

Life on Exoplanets? dwarfs, Kepler-22b around a G (white to yellow)-type star, and HD85512b around a K-type (orange dwarf) star. Our sun is a G-type star. After finding over seven hundred exoplanets, only these five meet the standards of being included in the habitable zone. Gliese 581g is about 20 light years away, and what makes it more favorable is that it is only 2.6 Earth Masses. Scientists are thrilled to see the brighter part of it approximately receives its starlight as much as our Earth does from our Sun. The estimated surface temperature is 10℃, and has a 32-day orbital period. However, though being a lot smaller than other candidates, it is still too large that the atmosphere would be dense and taken over by dramatic weather change. Also, it is too close to its parent star, but instead of getting burned, the tidally locked situation makes the matter worse. The side facing the sun would have the average temperature of 160℉, and on the other side it would be as cold as -25℉! Only a minimal fraction sitting on the edge between the two sides could be possible for life to exist. But people began to suspect Kepler-22b as a gaseous planet, since it has the volume between Earth and Uranus; HD85512b is thought to have the atmosphere similar to the early phase of Venus, according to its value of receiving 1.86 times to the sunlight Earth does, and therefore might cause a runaway greenhouse effect (with the ocean boiling in a certain period of time and gradually evaporated); the other Gliese planets are too large for us to live in, so technically speaking, Gliese 581g would certainly be our protagonist in the planet hunt.

III. Conclusion

The quest for Exo-Earths could be an unending journey, but with so many worlds undiscovered, there are more possibilities in the open. Hopefully with the help of better equipments and the boosting technology, we will at least find an exoplanet with life forms that are not that advanced in the near future.

IV. References

Solar System Quick (group). Exoplanets-The Search for Life. http://www.solarsystemquick.com/universe/exoplanets.htm Townsend, Rich. The Search for Extrasolar Planets. http://www.astro.wisc.edu/~townsend/static.php?ref=diploma-2 Schneider, Jean. The Extrasolar Planets Encyclopaedia. http://exoplanet.eu/ Worth, Mark. Exoplanets and Life in the Universe. http://www.physics.sfasu.edu/markworth/notes/lifenotes.htm Walter, F.M. Habitable Zone.

Life on Exoplanets? http://www.astro.sunysb.edu/fwalter/AST101/habzone.html Torres, Abel Mendaz. (2012). Five Potential Habitable Exoplanets Now. Retrieved Aug 1, 2012 from http://phl.upr.edu/press-releases/fivepotentialhabitableexoplanetsnow Rieke, G.H. “Properties of the Planets and Habitable Zones”. http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/habzone.htm Moskvitch, Katia. Goldilocks planet just right for life. Retrieved September 30, 2010 from http://www.bbc.co.uk/news/science-environment-11444022 Hook, Richard. (2012). Many Billions of Rocky Planets in the Habitable Zones around Red Dwarfs in the Milky Way. Retrieved March 28, 2012 from http://www.eso.org/public/news/eso1214/ Kaltenegger, L., Udry, S., Pepe, F. (August 18, 2011) Astronomy & Astrophysics. A Habitable Planet around HD 85512 ? Kitchin, Chris. (2011). Exoplanets: Finding, Exploring, and Understanding Alien Worlds. Springer Verlag.