Frequently Asked Questions

What is the measurement and how did you make it?

We observed the star upsilon Andromeda and its planet upsilon Andromeda b together (they look like a single point of light to us) at five different times over one orbit. We used an instrument called the Multiband Imaging Photometer for Spitzer (MIPS) on the Spitzer Space Telescope to measure the planet at a wavelength of 24 micrometers, which is a wavelength about 50 times longer than the human eye can see. At this wavelength, the star does not emit as much light as in the visible part of the spectrum, so it is favorable for measuring the dim planet. We measure variations of a few tenths of one percent in the system's total light (star plus planet). These variations have the same cycling as the known planetary orbit, and the amount of variation is consistent with physical models of such planets.

Shown at left is the measurement we actually made. The horizontal axis represents the phase of the orbit (that is, the planet makes a single orbit about the star as the phase goes from 0 to 1). At five times during the course of a single orbital period, we measured the brightness of the star+planet system. The points and associated error bars show how the brightness varied, relative to the value measured at the first epoch.

The solid line then shows the expected variation if the planet reradiates the absorbed energy from a hotspot as it goes around the star. This model is strongly constrained by the radial velocity measurements and the only parameter to be determined is the overall amplitude. Therefore, the agreement is very encouraging.

What are the main results of this measurement?

First, we have detected the difference between day and night on the planet upsilon Andromeda b, a non-transiting extrasolar planet. This is the first measurement of light coming directly to us from a non-transiting planet, and also the first measurement of light from the night side of any extrasolar planet. The specific pattern of this light tells us that the planet re-emits most of the energy it absorbs from its star immediately, as opposed to carrying it around to the night side and warming up the night. This is important because it tests theoretical models for how the atmospheres of extrasolar planets work. Second, we have made the first successful demonstration of a technique that can directly measure non-transiting extrasolar planets. This is important because most known extrasolar planets do not transit, and this is particularly true of nearby planets, which are the easiest to measure directly.

Why do you care, and why should we?

Astronomers care because of the possibilities for future measurements, particularly of smaller planets to be discovered in the future. Planetary scientists (including many astronomers) care because such a measurement tells us about the behavior of planetary atmospheres under exotic conditions. These are good reasons for you to care as well. A goal that many astronomers share with the public is to discover and measure Earth-like planets orbiting sun-like stars at distances where water can exist on the surface in liquid form. This measurement is a step in the direction of measuring such planets when they are found. Also, studying exotic atmospheres puts the Earth into context. As an analogy, who knows more about driving, someone who has driven the same sedan for his entire 20 years of driving experience, or someone who has spent 20 years driving everything from scooters to cars to big rigs to trains and boats?

Where does the name "upsilon Andromeda b" come from?

The star is in the constellation Andromeda. Bright stars are named in order of visual brightness within their constellation, using the Greek letters. Alpha Andromeda is the brightest star in Andromeda, beta is second, and so on. The 'b' indicates the first object detected in orbit around upsilon Andromeda. In this case the object is a planet, though the same naming convention is used for brown dwarf stars orbiting normal stars. The star upsilon Andromeda has three known planets, named 'b', 'c', and 'd'. Note that this naming convention goes in discovery order, not in order of distance from the star, so planets are not renamed when new planets are discovered in the same system. For more information, see here .

What's special about this planet?

The planet is a member of the class of "hot Jupiters". It is a non-transiting planet. Upsilon Andromeda b orbits in 4.6 Earth days and is located just about 1/10 as far from upsilon Andromeda as Mercury is from the sun. Its mass is about the same as Jupiter's, or a bit larger. We do not know its radius (since it does not transit, we cannot measure the radius), but theoretical expectations say it should not be dramatically larger or smaller than Jupiter.

What is a "hot Jupiter" planet?

Discovered in 1995, the "hot Jupiters" are perhaps the biggest surprise in modern planetary science. These are Jupiter-sized planets that orbit much closer to their stars than even Mercury in our solar system. There are no "hot Jupiters" in the solar system, but about 1/6 of extrasolar planetary systems known to date have them. The fraction of planetary systems that has them is probably lower than 1/6: these are the easiest planets to detect, so we detect more of them. For more information, see here .

How hot are they, really?

Recent measurements by some of the collaborators on this work show that these planets are indeed very hot, having atmospheric temperatures as hot as candle flames. Typical temperatures are in the range 1300 - 3100 degrees Fahrenheit (700 - 1700 degrees Celsius, or 1000 - 2000 Kelvin). These temperatures vary from planet to planet and even across the surface of individual planets, as we have shown by the present measurement.

What are the day and night temperatures of upsilon Andromeda b, exactly?

We cannot calculate the exact temperatures because we do not know the radius of this particular planet, nor the total amount of energy coming from it. We just know the difference in the energy coming from the day and night sides. However, assuming that it has about Jupiter's radius, the temperature difference between day and night is about 2550 degrees Fahrenheit.

Are "hot Jupiter" planets habitable?

These planets are thought to be like Jupiter, made of gas. So there is likely no solid surface where life could take hold. Furthermore, the temperature swings on this planet are extreme. Liquid water is a requirement for life as we know it and this specifies a particular range in temperature, somewhere between freezing and boiling. Gas moving across the atmosphere may pass through regions with the appropriate temperature range, but it will also cycle through temperatures where liquid water cannot exist. As a result, there is no location on any hot Jupiter planet where water can exist in liquid form for a long period of time. One can speculate about exotic forms of life that don't require liquid water, but we won't!

What is a transiting planet?

A transiting planet is one whose orbit takes it in front of (and also generally behind) its star once per orbit as seen from Earth. This geometry makes it much easier to measure those planets. From the transit (planet passing in front of star) we can measure the radius of the planet. From the eclipse (planet passing behind star) we can measure the amount of energy radiated by the planet in the infrared. Together these numbers (radius and emitted energy) give us a planetary temperature. Until now, only transiting planets have been directly measured.

What is tidal locking?

When a body rotates on its axis and orbits around another body at the same rate, it has the appearance of showing the same "face" to the body it is orbiting at all times. The Moon is a good example: it has a 29.5-day rotation rate and a 29.5-day orbit, so we always see the same side of it. Orbiting bodies do not start in this configuration. Rather, they raise tides on each other, and when they rotate the tidal bulges move to stay on the line between the two bodies. This causes friction, which dissipates energy, and the rotation of the bodies slows down. The moon has slowed all the way down and stopped, but the Earth has much more mass. Earth is slowing down (this has been measured) but will not stop within the expected lifetime of the solar system.

Why do astronomers think the hot Jupiters are tidally locked?

The same calculation of tidal friction vs. rotational energy that tells us the Earth will never stop rotating also tells us that hot Jupiters should stop rotating in just a few million years after they are formed. Since all these planets orbit stars that are a billion years old or older, it's pretty safe to say they are tidally locked, though this has not yet been observed.

Does tidal locking mean there are no winds on the planet?

No. Tidal locking refers to the planetary average motion. Surface winds carry relatively little mass compared to the whole planet. There can be strong surface winds so long as the planetary average is one rotation per orbit.

Where can I learn more about extrasolar planets?

All popular astronomy magazines regularly carry articles about extrasolar planets, too. Magazines like Sky and Telescope or Astronomy are examples. There are also a wide variety of astronomy clubs across the country and internationally. A partial list can be found on the Griffith Observatory website, which is obviously slanted towards Southern California, but does contain links for farther afield.

We like the Extrasolar Planets Encyclopedia . This site has a technical focus, but it summarizes all the measurements and scientific work. If you want to go straight to the source on any particular planet, or if you want to get lists of planet properties or generate plots of planet properties, this site is one-stop shopping. Some of the major observational programs also maintain websites where you can get up-to-date information. These include the California & Carnegie group and the Geneva group.

An excellent website describing the present and future NASA plans for studying extrasolar planets is PlanetQuest .

The underlying theme of the search for extrasolar planets is, of course, the desire to discover and study life on other planets. Much of the work on Astrobiology in the United States is funded through the NASA Astrobiology Institute .
Joe Harrington

Brad Hansen