The search for planets beyond our own Solar System proved to be a long, frustrating, and difficult endeavor–that is, until 1992 when the first exoplanets were successfully spotted circling a small, dense stellar corpse called a pulsar. Finally, in 1995, the first exoplanet was found in orbit around a main-sequence (hydrogen-burning), normal, Sun-like star. The exoplanet, 51 Pegasi b (51 Peg b, for short), was a “roaster”, a so-called hot Jupiter planet that hugged its parent-star, 51 Pegasi b, fast and close in a hellishly hot orbit. Planet-hunting astronomers have now succeeded in spotting many, many more exoplanets dwelling beyond our own Sun than the eight major planets that inhabit our Solar System. In December 2013, NASA astronomers announced that they had found dim signatures of water in the atmospheres of a quintet of remote exoplanets orbiting three different suns–and they did this using the venerable Hubble Space Telescope’s (HST’s) high-performance Wide Field Camera 3.
The distance to even the nearest star beyond our own Sun is so extremely vast that it must be measured in light-years. Light travels at 186,000 miles per second in a vacuum and, therefore, one light-year is six trillion miles! The nearest star to our own Sun is actually a triple star system named Alpha Centauri, which is about four light-years from our Solar System–or 24 trillion miles!
Many times during the last century, excited astronomers announced what they thought was the very first detection of a planet beyond our Sun’s family, and then looked on miserably as other astronomers failed to confirm their “discoveries.”
However, back in 1992, Dr. Alexander Wolszczan of Pennsylvania State University finally succeeded. Dr. Wolszczan, after carefully studying radio emissions emanating from a compact millisecond pulsar, dubbed PSR B1257+12–a dense little stellar relic dwelling in the Virgo constellation, which is situated approximately 1,300 light-years away–determined that it was being orbited by several very weird planets.
A pulsar is a relatively small sphere of approximately 12 miles, or so, in diameter. It is the collapsed core of what was once a massive main-sequence star that, after devouring its necessary supply of hydrogen fuel, perished in the brilliant fireworks display of a supernova conflagration. Pulsars, which are wildly spinning young neutron stars, contain up to 1,000,000,000 tons of star-stuff squeezed by gravity into a volume about the size of an apricot! Pulsars sport a density that is about 1,000,000 billion times the density of water!
51 Peg b
In 1995, Dr. Michel Mayor and Dr. Didier Queloz of Switzerland’s Geneva Observatory announced that they had discovered the first strong evidence of a planet orbiting a remote star that was like our own Sun. However, this great discovery left bewilderment in its wake. The newly discovered exoplanet, 51 Peg b, hugged its parent-star–51 Pegasi–very closely. At a mere 4,300,000 miles from its star, it orbits 51 Pegasi every 4.2 days!
51 Peg b is also a huge gas-giant, like Jupiter in our own Solar System!
However, then-existing theories of planet formation indicated that giant Jupiter-like planets must be born only at considerably greater distances from their stellar parents. So, what was the enormous 51 Peg b doing so close to its fiery parent-star?
In October, 1995, Dr. Geoffrey W. Marcy and Dr. R. Paul Butler confirmed the Swiss team’s discovery from the Lick Observatory’s three-meter telescope poised atop Mount Hamilton in California. 51 Peg b proved to be only the tip of the iceberg–it was the first discovery of an entirely new and unforeseen class of exoplanets termed hot Jupiters, which are enormous gas-giant planets that orbit their stellar parents in extremely close, roasting orbits. Since the discovery of 51 Peg b a generation ago, many other hot Jupiters have been spotted by planet-hunting astronomers in search of faraway worlds beyond our own Solar System.
New theories were developed to explain hot Jupiters. Some astronomers suggested that these “roasters” were essentially enormous molten rocks; while others postulated that they were gas-giant planets that formed about 100 times farther away from their stars and were boomeranged back through near collisions with other sister planets–or even by a companion star of their own stellar parent.
One particularly interesting theory is that hot Jupiters form at a distance comparable to Jupiter’s average distance from our Star, and then slowly lose energy due to interactions with the disk of gas and dust (protoplanetary accretion disk) from which they are born. The newborn giant planet, therefore, spirals into the inner solar system from its more distant, cooler place of birth.
Hot Jupiters may be doomed worlds, destined to crash to a flaming death inside the stellar furnaces of their murderous parent-stars. Until that time, these “roasters” orbit their stars fast and close, slowly cooking in their hellish orbits.
A New Exoplanet Quintet!
Using HST, two teams of astronomers announced in 2013 that they had spotted dim signs of water in the atmospheres of five faraway worlds.
The five exoplanets–WASP-17b, HD209458b, WASP-12b, WASP-19b, and X0-1b–all circle nearby stars. The strengths of their own water signatures vary. For example, HD209458b displays the strongest signal, while WASP-17b sports by far the puffiest atmosphere. The water signatures for the other trio of exoplanets are also consistent with the presence of water.
It’s not that water hasn’t been spotted before in some exoplanet atmospheres. However, what distinguishes these more recent studies, is that the astronomers’ objective was to conclusively compare and measure the attributes and intensities of signs of water on multiple exoplanets.
“We’re very confident that we see a water signature for multiple planets. This work really opens the door for comparing how much water is present in atmospheres on different kinds of exoplanets, for example hotter versus cooler ones,” explained Dr. Avi Mandell to the press on December 4, 2013. Dr. Mandell is the lead author of an Astrophysical Journal paper, published the same day, that describes the findings for WASP-12b, WASP-17b, and WASP-19b. Dr. Mandell is a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
The studies of this bequiling, watery quintet of alien worlds, were part of a large census of exoplanet atmospheres that was led by Dr. L. Drake Deming of the University of Maryland in College Park. Both teams of scientists used HST’s Wide Field Camera 3 to study in detail the absorption of light through the atmospheres of those distant worlds. The studies were accomplished in a range of infrared wavelengths where the water signature–if it was there–would show up. The teams then compared the intensities and shapes of the absorption profiles. The consistency of the signatures indicated to them that they were seeing the presence of water. The observations also highlight HST’s continuing value in exoplanet research.
“To actually detect the atmosphere of an exoplanet is extraordinarily difficult. But we were able to pull out a very clear signal, and it is water,” said Dr. Deming to the press on December 4, 2013. Dr. Deming’s team reported their results for the two planets HD20945b and XO-1b in a September 10, 2013 paper, also published in the Astrophysical Journal. Dr. Deming’s team used a new technique that employed longer exposure times. This increased the sensitivity of their measurements.
In order to discover the composition of an exoplanet’s atmosphere, astronomers observe the planet as it transits (passes in front of) its glaring parent-star. The scientists then look at which wavelengths of light are being transmitted and which are partly being absorbed.
The signatures of water were all less intense than the astronomers had expected, and they now think this may be due to a layer of dust or haze that cloaks each of the five exoplanets. This veiling cloak of dust or haze can diminish the intensity of all signals emanating from the atmosphere–in much the same way that fog on our own planet can mute the colors in a photograph. Also, the veil of haze can change the profiles of water signals, as well as that of other important molecules, in a particular way.
The quartet of worlds are all hot Jupiters–just like 51 Peg b, the very first exoplanet to be spotted in orbit around a Sun-like star. The astronomers were at first surprised that all five of the exoplanets seemed to be veiled in haze. However Dr. Deming and Dr. Mandell both noted that other scientists are also discovering hazy veils around distant worlds.
“These studies, combined with other Hubble observations, are showing us that there are a surprisingly large number of systems for which the signal of water is either attenuated or completely absent. This suggests that cloudy or hazy atmospheres may in fact be rather common for hot Jupiters,” Dr. Heather Knutson told the press on December 4, 2013. Dr. Knutson, of the California Institute of Technology (Caltech) in Pasadena, is a co-author on Dr. Deming’s study.
Dr. Geoffrey Marcy, who has spent most of his career searching the depths of Space for exoplanets, has more recently decided to alter his focus, and search for something just a bit more precious–and elusive: extraterrestrial intelligence. Dr. Marcy, who was recently named the Watson and Marilyn Alberts Chair for SETI at the University of California at Berkeley, is a pioneer in the search for planets beyond our own Sun. His work has resulted in the discovery of more than 110 exoplanets–including the first system of worlds orbiting a distant star.
Dr. Marcy, along with several other pioneering astronomers who were the first to spot exoplanets, is pegged to win a future Nobel Prize. He said in a recent interview with George Dvorsky:
“I would love to know if other intelligent beings exist elsewhere, and if so, is our nearest neighbor a few light years away or thousands of light years away… (W)e should keep any open mind, answering the dry, cold, and frightening question, ‘Are we alone?’ An answer either way, yes or no, will be shocking for we Homo sapiens.”