Other Stars Like the Sun (cont.)

One more time, a mention of the star most like the Sun of all we know. 18 Scorpii, or HR6060.

>>>>>> Abstract from the Astrophysical Journal.

In the future, we will find many more as our measurements of distances become precise for more distant stars.

Planets around other stars: the Hot Jupiters

Nature usually turns out to be stranger and more interesting than our expectations.

Mentioned last time that the stars are too far away to detect terrestrial planets. Until a few years ago, we did not know of any planets beyond the solar system. That has changed drastically since 1995.

For 10 years or more, a couple of programs were underway to detect planets by the changing Doppler Shift of the star as it moved about the center of mass of the star-planet system.

>>>>>> blackboard drawing with Doppler shift.

The complete cycle of a periodic Doppler shift would have the period of the major planet….12 years in the case of Jupiter, 29 in the case of Saturn. In the early 1990’s astronomers were considering themselves as beginning this type of measurement for definite detections decades in the future.

In 1995, the astronomical world was greatly surprised by the announcement that a group at the Observatory of Geneva had already discovered a planet around the solar-type star 51 Pegasi. 51 Pegasi is almost directly overhead in the early evening now, although you need a star chart to find it.

What was astonishing was that this planet would orbit the star in a few days. Once astronomers knew what to look for, they started to discover lots of them. The table in your textbook, current as of February 1999, lists 20 planets outside the solar system. The most up-to-date list is on the web at the URL given below.

http://www.obspm.fr/encycl/catalog.html

Check the main attributes of these planets.

1. Most are comparable to the mass of Jupiter, or larger.

2. Their distances from the star are way under 1 astronomical units. Almost none are at the distances of 5 – 10 au that we find in our solar system.

Notice that many of these planets are about 0.05 astronomical units. That is about 10 times the radius of the star. And these seem to be planets like Jupiter! Let’s look at this in graphical form.

http://cannon.sfsu.edu/~gmarcy/planetsearch/multi_panel.jpg

This is the lesson we get from these observations. In all of these solar-like star systems, there are planets, apparently similar to Jupiter and Saturn, but located far inside the orbit of Mercury.

Why is this a very surprising discovery?

To resolve this paradox, we have two choices:

Our ideas about how the planets formed in our solar system are wrong.
The Jovian planets in these solar systems formed far out, as our theories say they should have, but some process moved them in.

We currently believe it is the second of these possibilities, and there are a couple of possible processes which could have caused “migration” of these giant planets. One of the more compelling is the formation of additional massive planets further out in the early planetary nebula would have “shepherded” the Jupiter and Saturn-like planets into the Sun.

The big question which arises now is the following. Are these systems with the “hot Jupiters” anomalous (even though there are lots of them now), or is the solar system anomalous in that the Jupiters never marched through the inner solar system on their way to the Sun?

One thing that should be emphasized is that the detections to date may be examples of a “selection effect”. The measurements will find the star systems which have such planets, but would not indicate star systems with “normal” planetary systems.

An Intriguing Suggestion

These finding have produced a recent intriguing suggestion about the history of terrestrial planet formation in the universe.

· As time goes on in the history of the universe, the abundance of heavy elements (oxygen, carbon, nitrogen, iron, etc) increases. These are the elements which are necessary for the precipitable material that makes up the planetesimals.

· Early in the history of the universe, the abundance of the heavy elements was too low. The mass of material that could be precipitated out of the protoplanetary nebula was too small to form planets of the sort we see.

· Late in the history of the solar system, the abundance of heavy elements was too high. There was too much material to precipitate out, and the large number of planets in the solar systems would have caused migrations of Jovian planets from the outer solar system into the Sun.

· As a result, there was an optimum period to form terrestrial planets in stable solar system.

C.H. Linewater estimates that 75 percent of star systems that would form stable terrestrial planets did so before the Sun formed. Linewater estimates that the average terrestrial planet in the universe is 1.8 billion years older than the Earth. If Linewater is correct, the Earth is a latecomer in the universe. Look out at the night sky and think that it might be filled with Earth-like planets much older than the Earth. This idea will be of great significance in the next and final topic in this course.