We begin with a quote:
Our Solar System is a majestic place of great diversity. Here is Jupiter, with its volcanic moon Io and its icy -- and perhaps watery -- moon Europa. Image from Voyager 1 (JPL).
The barren rockiness of Mars looks almost like something we might find on Earth. It looks very different from Jupiter. This image is from Mars Pathfinder (JPL).
This exotic looking photo actually shows the South Pole of the Earth, as seen by the Galileo speacecraft (JPL).
And this foreign looking body is also the Earth, as seen from the Clementine spacecraft. This is called the "limb" of the Earth. You can see the vertical structure of the atmosphere, as well as a "glint" of the sun off of the atmosphere. (Naval Research Lab)
Here we see the Earth and the Moon, lit from the side, as seen by the Galileo spacecraft, on its way to Jupiter. Planets around other stars may look like Jupiter, or more like Mars or the Earth, or the Earth's moon ... or like none of these. (JPL)
Planets around other stars may look like what we are used to, or like nothing we have ever seen before. (Photo by D.T.)
And, of course, among the things we are interested in are the presence of water -- and potentially life -- on extrasolar planets (planets which orbit other stars). (Photo by D.T.)
To learn about other planetary systems, we first learn about our own.
This montage of the planets of our Solar System shows that planets come in all sizes and compositions in our Solar System. What governs how big planets can get? What governs how many planets there are in a planetary system? These are questions we want to answer. (JPL)
Saturn is the most easily recognizable planet in our Solar System, with its majestic ring system, seen here by the Voyager spacecraft. (JPL)
The Galileo spacecraft revealed to us the artistic beauty of Jupiter's swirling atmosphere. This false color image of storms in Jupiter's atmosphere shows that Jupiter experiences weather on a dramatic scale. Some storms on Jupiter have lasted since before Galileo (the man) invented the telescope: the Great Red Spot was one of the first features that Galileo described on this giant planet, and this storm persists to this day. Here we see the white ovals, which have persisted since the 1930s. (JPL)
This is a topographic map of Mars (data from Mars Global Surveyor). Broad low and high elevation regions on Mars are evident, reminiscent of the Earth. Mars (and the Earth, and Venus and Mercury) all are quite different than the outer planets (Jupiter, Saturn, Uranus, and Neptune), in that the inner planets are largely rocky, and the outer planets are largely gaseous. Planets around other stars could be either, or of more than one type. (JPL)
Another picture of Mars from Mars Pathfinder. The scene looks much like the Earth, despite being a foreign planet. Will extrasolar planets remind us of the Earth, or be quite different? (JPL)
Venus is the planet often referred to as Earth's twin. Venus' size is very similar to the Earth's, but its surface is much hotter, and would not be a friendly place for life as we know it. Shown here is Sapas Mons, a 1.5 km tall mountain on Venus. Maat Mons (8 km high) is in the background. Data from the Magellan spacecraft. (JPL)
At last, a place we know intimately. Seen from space, this view of the Bay Area looks familiar, yet somehow foreign as well. What do we learn about our own planet in the process of space exploration, while we are searching for other planets? And what will we learn about our Solar System's planets, in comparison to planets around other stars? (Space Shuttle image, from JPL)
This image shows 12 views of the Sun, taken over 4 years, by the Soft X-ray Telescope (SXT) on the Japanese Yohkoh satellite. The Sun, of course, heats the planets in our Solar System, and is what allows life to exist on Earth today. When people look for planets around other stars, and when in the future we search earnestly for life elsewhere in the universe, we will have to study specific stars to know where to begin. Studying specific stars is also the key to finding planets around other stars, as described below. "Extrasolar planets" are planets which orbit a star other than our own Sun. (SXT/Yohkoh/ISAS)
People are looking for -- and finding -- planets around other stars, discoveries that are changing the way we think about the evolution of planets, planetary systems, ... and life.
The "radial velocity" technique is a powerful tool which identifies stars which may have planets orbiting them. In the radial velocity technique, small gravitational effects of the planet on the star are measured by powerful telescopes, and from the character of the signal, details of the planet are determined. (G. Marcy)
This figure shows the wobble of OUR Sun, if it were to be observed by elsewhere in the galaxy. This is the kind of motion which we must detect to find extrasolar planets by direct detection. The motion is equivalent to my holding up a quarter here in Santa Cruz, and someone in Vancouver seeing it. Or, like reading a computer screen from 10,000 miles away. (NASA/ExNPS)
Here is some data for a star called 51 Pegasi, a star which is roughly the 51st brightest star in the constellation Pegasus. This star was the first star like our Sun which was shown to have a planet orbiting it. The data shown here seem to be scattered a lot ...
...but when the data are shown in phase, a regular pattern reveals itself. This pattern is the motion of the star, as it wobbles back and forth in gravitational reflex to the planet's orbit around the star. What's unusual about this planet's orbit (its year) is that it is only 4.23 Earth days long! (All data from G. Marcy)
This figure shows all of the known extrasolar planets, to date. In fact, almost all of these extrasolar planets have very short orbits, compared to Jupiter's orbit of 11 years. The Earth's orbit, of course, takes 1 year (corresponding to 1 AU on this figure) -- but some of these extrasolar planets orbit their star in only a few days! (G. Marcy)
As you probably have heard, many of the detections of extrasolar planets have been made here at Lick Observatory. Geoff Marcy, Paul Butler, Steve Vogt, Debra Fischer, Michael Liu, and others have discovered more than a dozen extrasolar planets, many of those from here at Lick. (Lick)
This same team has also used the Keck telescopes in Hawaii to discover extrasolar planets. Shown here are the two Keck telescopes, on the summit of Mauna Kea, on the Big Island of Hawaii. In the background is the island of Maui. There are two people standing on the dome on the left, to show the scale! The two Keck telescopes (10 meters each) are the largest telescopes in the world. (Keck)
In addition to discovering extrasolar planets with the radial velocity technique, we also get clues about other planetary systems by looking for disks around other stars. Disks, it is thought, indicate either locations where planets may be forming, or locations where planets have already formed. Although we do not know whether every disk implies planets, the connection may be strong. Shown here are several disks around various stars. These disks have been imaged in various wavelengths. We learn about the properties of these disks by studying them at various wavelengths. (P. Kalas)
Here is another picture of a disk, taken at the Keck telescope. You may have seen this picture on the cover of Newsweek. What is special about this disk is that there is a hole in the middle of the disk: what we actually see is a donut of disk-dust. This hole could be caused by the presence of a planet in this system (although there are many other possible causes for this hole). (NASA/JPL/MIRLIN/D. Koerner)
In looking for other planetary systems, we also look in areas of the galaxy where very young stars are being born. This image, from the Hubble Space Telescope, shows giant pillars of gas which will ultimately collapse and form young stars. Presumably, conditions in these early stages help dictate how and how often planets will form. This image has been in the press a lot -- you may have seen it at the very beginning of the movie "Contact," for example. (NASA/STSCI)
Something else that we are very interested in doing is taking pictures of planets -- directly detecting them. The radial velocity technique, while powerful, is only an indirect measure of a planet, since what is observed is a characteristic of the star. This image shows what might have been the first direct image of a planet. It has likely turned out that this is not a planet, but a star -- sometimes it's hard to tell the difference! However, a young, hot, glowing planet might look like this. (S. Terebey, Extrasolar Research Corp., NASA/STSCI)
The real goal is to learn about other planetary systems and our own by comparing what we know about each.
This figure shows the various orbits for bodies orbiting stars. Can you pick out which one is our Solar System? From this, we maybe can learn what kinds of orbits are typical, and what are unusual. (Casey/J. Schneider)
Here are the orbits of several of the extrasolar planets, and the Earth's orbit. Some are similar ... and some are very different. Also, all the extrasolar planets represented in this image are very massive -- Jupiter's size, over 300 times bigger than the Earth! (G. Marcy)
This figure shows a comparison between the only (to date) known planetary system with more than one planet in it, other than our own. These planets, orbiting Upsilon Andromeda, were discovered at Lick and Keck. Our Solar System's planets are shown in comparison. The orbits are similar, but the Upsilon Andromeda planets are much more massive. (A. Contos/CfA)
Beta Pictoris is the star with the best-known disk, shown here. The size of the disk around Beta Pictoris is shown, in comparison to Pluto's orbit. The Earth and the rest of the planets in our Solar System are much closer to the Sun than Pluto is, and would be within the blacked-out parts of these images. Our Solar System is also surrounded by a disk of dust, although it is much smaller than the disk around Beta Pictoris. These images are from the Hubble Space Telescope. (NASA/STSCI)
What does the future hold for finding and characterizing extrasolar planets?
One of the planned improvements will be the advent of the Keck Interferometer, which will utilize the existing Keck telescopes, on Mauna Kea, and in addition use several smaller, outrigger telescopes. The interferometer will work by combining the light from all these telescopes to effectively make one very large telescope. This sketch shows how the Keck Interferometer will work. The intereferometer is scheduled to be completed by 2003. (Keck)
Another of NASA's future plans includes building the Terrestrial Planet Finder (TPF). This spacecraft will consist of a flying formation of telescopes, and will work in a similar way to the Keck Interferometer. TPF will have the advantage of flying in space, and its goal will be to find planets like the Earth. Launch is scheduled for 2011. (NASA)
The European Space Agency (ESA) has a similar plan, called DARWIN, which also will look for Earth-like planets. Launch in 2009. (ESA/DARWIN)
In the future, we will no doubt find many more planets like those that have been found already, which are similar to Jupiter. And, probably in the not-to-distant future, we will start to take pictures of these Jupiter-like extrasolar planets. (NASA/Voyager)
But of course, the holy grail of planet searches will be the first images of an Earth-like planet. Years in the future, we might look for medium-resolution pictures of terrestrial planets to look like this. (NASA/Origins)
And we may find many planets which look nothing like the planets we know ...
... or they may look very similar. And, of course, we would like to look for the beginnings of life, too. (Both photos by D.T.)
A final quote to leave you with.
