Colloquia November 8

On November 8, 2012 the University of Michigan welcomed back a former graduate student Zhaohuan Zhu. 

Zhaohuan Zhu was a graduate student at the University of Michigan some years ago. He now students at Princeton.

Zhu focused of the fluid dynamics of planetary system formations. He described why he believed it was better to used 3D over 2D simulations. He said it was better because we could see more data about the way a planetary system works.

He went into great detail about how planets form. He talked about using radio velocities techniques (measuring the wobble of the planet) and imaging (viewing the planet head on).

Zhu also showed a very interesting video about the Almer telescope and its array formation. It is composed of many radio telescopes to give astronomers a deeper clearer view of the universe.With the Almer telescope astronomers will be able to view deeper into space as far back as many radio waves, and since they are in an array they are not limited by the viewing power of the telescope itself. They are all put together as one so essentially the viewing power is only limited by the amount of telescopes in the array.

Planetary System Formations

Planetary System Formations

To understand how planetary systems form I will focus on the Solar system because it is the most widely studied system. Astronomers believe the nebular hypothesis when it comes to the solar system. They believe that the solar system formed from the collapse (gravitationally) of a portion of a very big molecular cloud. The formation of the Solar system occurred about 4.55 billion years ago

The molecular cloud was most likely about 20 pc and the part that actually collapsed to form the solar system was about 1 pc or 20,000 AU in size. 

Here is a picture of a molecular cloud:

The 1 pc region included a mass of a little bit more than the Sun (around 1.98*10^30 kilograms). Hints: the sun is the most massive object in the Solar system. This region was composed of primarily Hydrogen and Helium with very tiny amounts of lithium.

The molecular cloud at a certain point began to spin very fast because of angular momentum. The atoms inside cloud began to collide and they converted their kinetic energy into heat. As it continued to collapse the center of it was much hotter than is surrounding disk. After about 100,000 years the forces of gas pressure and gravity competing led to the formation of a protostar . After 50 million years the protostar became hot enough to fuel itself through nuclear fusion and the protostar became what is known as the Sun.

The planets in the solar system formed from the disc shaped cloud containing dust and gas that the sun left after its formation. Astronomers believe that the planets (like the earth) began as grains of dust and accumulated matter over years until they became planets. This process is very inefficient according to Astronomers when compared to star formation. While the gas giants in the solar system formed much farther out.

What is a Planetary System?

Astronomers have studied the Solar system for many years and because of this they know that planetary systems take 1 to 10 million years to form.

Here is a general picture of what a planetary system could looks like:

A planetary system is a collection of gravitationally bound celestial objects that orbit around a star or a system of stars.  These systems vary in sizes and vary in the amount of planets they contain.  Astronomers have frequently discovered single planetary systems using radial velocity method calculations.

Planetary systems usually describe systems with one or two planets and a star, but these systems can contain multiple stars, multiple planets, satellites, dwarf planets, asteroids, meteoroids, and comets.

Astronomy Colloquia 11/1

On November 1, 2012 the Astronomy department at the University of Michigan held a Colloquia. Jason Wright, an assistant professor of astronomy and astrophysics at Pennsylvania State University, was the main speaker.

Wright is a member of the Center of Exoplanets for Habitable Worlds and the Penn State Astrobiology Research Center (part of the NASA Astrobiology Institute). He study stars, their atmospheres, their activity and their planets.

During his speech he focused on the detection and the discovery of exoplanets.  Exoplanets are planets that are discovered outside of the solar system. for more info on exoplanets go here: http://www.michastrostudent.blogspot.com/2012/11/exoplanets.html. His speech was very interesting because it described the indirect (for more information:http://www.michastrostudent.blogspot.com/2012/11/indirect-exoplanet-detection.html)  and direct (for more information: http://www.michastrostudent.blogspot.com/2012/11/direct-exoplanet-detection.html) methods of detecting exoplanets in detail.  He also described how hard it is to detect a habitable planet and the key components that define an exoplanet as habitable or non habitable. 

Direct Exoplanet Detection

There are two methods used to directly detect exoplanets.

The first direct method used is referred to as imaging. Planets are light sources, although sometimes very faint light sources. To discover exoplanets using this method observers can see light produced by an exoplanet. Using this method is extremely difficult because older or middle aged exoplanets produce very little light, especially if they are small. This method has usually only worked when observing hot young exoplanets. The light produced by the exoplanets' companion star can literally out shine the light produced by the exoplanet and the exoplanet can go undetected. 

 

The second direct method is infrared interferometry. Traditionally telescope's viewing power is limited by the diameter of the telescope's mirror or lens, but combining telescopes in an array can greatly boost a telescopes viewing power. Array telescope in space can then use infrared interferometry to detect exoplanets and their companion stars. This method is the newest method of detecting exoplanets but it seems to be the most promising because array telescopes could potentially easily detect exoplanets that take years to detect using other methods.

Indirect Exoplanet detection

 There are four indirect methods used to detect exoplanets.

The first method indirect is the radial velocity method. It is the most common method used to discover exoplanets. The reflex motion of a star due to the orbiting planet is measures as a change in a stars radial velocity. The radial reflex of the star is compared to the exoplanets orbit, using the measurements of Doppler shifts. These comparisons are used to calculate the mass of the exoplanet, its orbits shape, and its orbital distance. The exoplanets discovered using this method tend to be very low mass planets. 

 The second indirect method is the astrometry method. This method measures a star's position and how it changes over time so is mostly used to discover exoplanets that have very long periods.  After that you can use the acquired information to determine the actual mass of the exoplanet because you can determine the orbital plane of the exoplanet. The best place to use astrometry is in space but you can use this method from the surface of earth. The exoplanets discovered using this method tend to be very far from the solar system. 

 

The third indirect method is the transit method. A transit is an event that occurs when a celestial object moves in-front of another celestial object. When the celestial body moves infront of the other larger celestial body it hides a small portion of it. Observers can see this occurrence at particular orbital points. This method reveals exoplanets when they transit their larger companion stars. Observers see a drop in the visual brightness of the companion star. The exoplanets orbit has to be perfectly aligned with the observers viewing point or the observer could easily miss the exoplanet. Also there is a very high amount of false exoplanet detections when using the transit method because dust, gas, and even planetary debris can easily cause a star to appear dimmer. 

The fourth indirect method is gravitational lensing. Gravitational lensing occurs when the presence of matter effects the path of a light ray. The light ray, from the observers view can appear to be curved or highly unusual. The gravity field of a star can behave like a lens and it can magnify the light of a background star. The star, the background star, and the Earth all move relative to each other. If the lensing star has a companion exoplanet, then the exoplanet's gravitational field can be detected through its contribution to lensing effect. This effect only occurs when the stars are almost perfectly aligned. The lensing events are very short and they can never be repeated so it is very difficult to detect exoplanets using this method.

Exoplanets

An exoplanet is a planet that is found outside of the solar system.  Exoplanets are also referred to as extrasolar planets.

Astronomers use many different techniques to locate these planets. Astronomers have discovered 843 exoplanets, 663 are in single planetary systems and there are 126 exoplanets in multiple planetary systems. Astronomers have predicted that there are above a billion exoplanets in the Milky Way galaxy.

Below is an example of an exoplanet orbiting in a binary star system:

To discover exoplanets astronomers use three techniques. The first technique involves using precise radial velocities, and this technique is the most commonly used. The second technique is the transit method, and the third is imaging.  Imaging is the hardest method of discovering exoplanets used be astronomers.


Here is a picture of some of the discovered exoplanets compared to their companion stars: