# Exoplanets

Exploring the universe

## How do we locate stars

Astronomers are using something called sidereal time. Exoplanet hunter is using sidereal time to calculate the position of stars in the sky relative to the user on earth. Earth is rotating one revolution around its axis in 24 hours and 0 minutes in that time the earth has moved approximately one degree on the path around the sun. Therefore relative to the distant stars the earth day is just 23 hours, 56 minutes and 4 seconds. As the day is shorter the time will back every day so how do we calculate sidereal time?

It could be easily done using this formula[1]:

Where d is the days that passed since new years eve and tUTC is the local mean time. The time is using UTC that is the common time standard across the world. Sidereal time is the time displayed in the star map of the app.

In mathematics and in many applications in physics it is common to use a spherical coordinate system for 3D space instead of a Cartesian (x,y,z) system. the polar angle or the altitude is the angle between the direction where the mobile user is pointing his/her phone at the sky and the xy-plane.

The azimuth angle is the angle between the x and y-axis on the plane and r is the distance from the origin. Expressed in Cartesian coordinates:

When activating the orientation sensors in the phone (GPS sign) the parameters alt (altitude) and az (azimuth) are shown. The angles are based on the Euler angles that describe the orientation of a rigid body, with respect to a fixed coordinate system. Where the point of origin is the user holding the phone. Stars in the sky have fixed coordinates, right ascension, and declination. The point of origin for those coordinates is a point in space. Earth is orbiting the sun and the thought path for that orbit is called the ecliptic. Earth is also rotating within a thought geocentric celestial sphere. The point of origin is where the equator of that sphere is crossed by the ecliptic line. With reference to that point right ascension is the azimuth angle, it is measured within an hour circle from 0 hours to 24 hours. Declination is the polar angle with respect from the same point and, is measured in degrees -90 to 90.

To calculate right ascension and declination for a user could be tricky considering that earth's surface is curved and that Euclidean geometry cannot be applied. The variables in the equations are the user's geographical position and sidereal time. For the interested here comes the formulas[2]. The hour angle h.

Where φ o is the latitude of the observer holding the phone. It is recommended that one is using the two-argument arctangent to compute the arctangent of the quote. Right ascension can now be calculated using the sidereal time.

And the declination can be calculated using the formula:

## Gliese 581g the earth twin that disappeared

In the constellation Libra 20 light years there is a star with a interesting and important planetary system. The star called Gliese 581.To find it in the Exoplanehunter app you need to search for the name GJ 581. Gliese 581 a red dwarf and is a third of the mass of the sun.

The first planet that was discovered orbiting Gliese 581 was Gliese 581b in August 2005 a neptunian.

The second planet discovered in the system was Gliese 581c in April 2007. The planet has a mass is bigger than Earth a so called Super Earth.

Gliese 581 c was chosen for the message from Earth. From Ukraine's National Space Agency  a high-powered digital radio signal was sent to the planet in october 2008 it will reach its destination first in 2029. The message contains images of landmarks,famous people like Hillary Clinton and notes written by Bebo members. Later it has been confirmed to be too hot for life.

Another planet was discovered in 2007 in the star habitable zone but much more massive than earth Gliese 581d. Also a message was sent to Gliese 581d. A website "Hello from Earth" run by Cosmos magazine collected messages. The messages were later send from the DSS-43 70 m radio telescope at the Canberra Deep Space Communication Complex at Tidbinbilla, Australia, A study from 2012 concluded that Gliese 581 d  is likely to be an result of stellar activity and does not exist.

In April 2009 the third confirmed planet was reported Gliese 581e that was discovered by an Observatory of Geneva team led by the Swiss astrophysicist Michel Mayor. Gliese 581e has similar size as Earth but is even closer the star than Gliese 581c.

At the end of September 2010, Steve Vogt a astrophysicist from UC Santa Cruz discovered Gliese 581 g he nicknamed the planet Zarmina after his wife. The discovery was a huge sensation at the time as it was the first earth-like exoplanet discovered in the habitable zone of a star.

If you visit the GJ 581 in the app you will not be able to see the planet. It because the planet has not yet been confirmed and studies from 2014 have shown that the planet probably does not exist. According to PHL Gliese 581 d and g do not exist

• Gliese 581 c (too hot)

• Gliese 581 d (does not exist)

• Gliese 581 g (does not exist)

Researcher Paul Robertson studies concluded as Gliese 581g was detected from Gliese 581d orbit and Gliese 581d was false positive due to stellar activity. Other studies from by a research team led by Guillem Anglada-Escudé in 2015 claims that Gliese d could exist despite the stellar activity and that the data should be reanalyzed.

Hypothetical say that  Gliese 581 g does exist, how similar to earth is the planet and could there aliens? Gliese 581 g is very close to it sun. The planet will not rotate around it axis and always have the same side pointing toward it sun. On one side of the planet it is always day and on the other side always night. The atmosphere i dense and could support life if the planet has liquid water. Temperature between −31 °C and -12 °C or −12 F and 10 F as average could be hot on the day side and the most habitable area on the planet is between the line of day and night.

## How to Classify Stars

There are different types of stars. Stars are usually classified based on their spectral characteristics. There are seven different types O, B, A, F, G, K, and M. Researchers some times use mnemonic to help them rember the order like
Oh Be A Fine Girl (or Guy), Kiss Me

Type

Color

Temperature

$\fg_c6d4ff M\odot$

$\dpi{120} \fg_c6d4ff L\odot$
O Blue > 25,000 K 60 1,400,000
B Blue 11,000-25,000 K 18 20,000
A White 7,500- 1,000 K 3.2 80
F Yellow 6,000-7,500 K 1.7 6
G Yellow 5,000-6,000 K 1.1 1.2
K Orange 3,500-5,000 K 0.8 0.4
M Red < 3,500 K 0.3 0.04

K = Kelvin 1 Kelvin = -273 ° C or -459.67 Fahrenheit, it is the absolute freezing point colder can it not be, atoms would stop moving. Stars are also classified by luminosity which is the amount of energy that a star emits per unit of time. Luminosity is measured in joules per second or watts just like power. It is a measure of the brightness of the star and is usually counted in comparison with the sun $\dpi{120} \fg_c6d4ff L\odot$

The solar mass is a standard unit of mass in astronomy is  denoted $\dpi{120} \fg_c6d4ff M\odot$ where $\dpi{120} \fg_c6d4ff {\odot }$ is the sun symbol,

 Type Star Ia luminous supergiants Ib less luminous supergiants II bright giants III normal giants IV subgiants V dwarf VI subdwarf VII White dwarf

Hertzsprung-Russell diagram

A famous diagram in astronomy is the Hertzsprung-Russell diagram.
The diagram is a plot of luminosity against the temperature of the star.
Both luminosity and temperature are proportional to the star mass only. Therefore stars of different mass will lie on a line this is known as the main sequence. As the stars spectral characteristics is a good indicator of temperature it is used on the horizontal axis of the diagram

Our sun is a main sequence star of class G2V (yellow dwarf), which means it has a medium temperature and normal size.

## How do we classify planets

There are different types of planets and different ways to classify planets. One way is to arrange planets after their mass. Gas giant is called Jovian and is a massive planet with a thick atmosphere and a dense liquid core. In our solar system are Jupiter and Saturn Jovians. Neptunians are planets of the same order as Neptune and Uranus. A super-earth (also called Super-terrestrial) is a planet that is significantly larger than Earth but less than Neptune's about 10 times as large as Earth. A terrestrial planet is a planet of the same size as the Earth.

Planets are also arranged after composition: gas, water-gas, stone-water, stone-iron or iron. Or by temperature, warm, habitable or cold planets. Among extrasolar planets there is something called Hot Jupiter, it's a gas giant that orbit near its star and therefore has a very high surface temperature. These planets are the most common extrasolar planets, because they are relatively easy to detect.

There are seven different criteria that the researchers use to classify planets that can contain extraterrestrial life:

• Earth Similarity Index (ESI) How similar to a planet is Earth on a scale between 0 to 1 where 1 resembles Earth most. ESI is based on the radius of the planet, density, air velocity and surface temperature.

• Standard Primary Habitability (SPH) how suitable a planet is for vegetation on a scale of 0 to 1 where 1 is most likely to be possible for vegetation. SPH depends on the surface temperature of the planet

• Habitable Zone Distance (HZD) distance from the center of the star's habitable zone so that -1 is in the outer zone and 1 is in the inner zone.

• Habitable Zone Composition (HZC) The planet's composition values ​​below -1 mean that the planet consists mostly of iron and values ​​above 1 that the planet consists mostly of gas. Values ​​close to 0 consist mostly of stone-water-iron.

• Habitable Zone Atmosphere (HZA) is based on the possibility that the planet has a habitable atmosphere, the values ​​below -1 mean that the planet lacks atmosphere and values ​​above 1 that the planet has a very thick atmosphere and is probably a gas planet. Values ​​close to 0 are not necessarily ideal for habitable atmosphere.

• Planetary Class (pClass) is based on the planet's mass or temperature zone.

• Habitable Class (hClass) Classifies planets by Temperature Hypopsychroplanets Cooler Than -50 ° C, psychroplanets Cold (-50-0 ° C), Mesoplanets Normal Temperature (0-50 ° C), Thermoplaneter Warm (50-100 ° C), Hyperthermoplanets warmer than 100 ° C.

## Discovery methods of exoplanets

### Micro lensing

Even massless photons is affected of gravity, because they have both mass and momentum. It is predicted by Einstein's general theory of relativity that the light will bend when passing an heavy object to follow the curvature of spacetime, it is the closest and most natural path for the light to take. This theory was proven with experiment during a total solar eclipse in 1919 by Sir Arthur Eddington. Similar methods are used to detect exoplanets.

By looking on a distant star, let us call that star for Star A. Star A is far away from another star called Star B, but is aligned behind it when observed by telescope from earth. It will lead to a deformation of light in to two distorted images of Star A. If there is an planet orbiting Star B it produces a third image of Star A.

When light from Star A pass on all sides of Star B it produces a ring of light around the object and that phenomena is known as an Einstein ring.

### Transit method

Another method of detecting exoplanets is to measure the magnitude of the star over time. When an exoplanet pass in front of the star there will be a small drop in brightness. This drop will occur for every revolution of the planet. The decrease is very small depending on the planet mass, usually between 0.01 percent and 1 percent.

As the star's mass and size can be determined from spectroscopic analys of the observation then also the mass and the distance of the exoplanet can determined.