15.1: Exploration

Learning Objectives

By the end of this section, you will be able to:

• Describe how the observations of protoplanetary disks provides evidence for the existence of other planetary systems
• Explain the two primary methods for detection of exoplanets
• Compare the main characteristics of other planetary systems with the features of the solar system

Until the middle 1990s, the practical study of the origin of planets focused on our single known example—the solar system. Although there had been a great deal of speculation about planets circling other stars, none had actually been detected. Logically enough, in the absence of data, most scientists assumed that our own system was likely to be typical. They were in for a big surprise.

Discovery of Other Planetary Systems

In The Birth of Stars and the Discovery of Planets outside the Solar System, we discuss the formation of stars and planets in some detail. Stars like our Sun are formed when dense regions in a molecular cloud (made of gas and dust) feel an extra gravitational force and begin to collapse. This is a runaway process: as the cloud collapses, the gravitational force gets stronger, concentrating material into a protostar. Roughly half of the time, the protostar will fragment or be gravitationally bound to other protostars, forming a binary or multiple star system—stars that are gravitationally bound and orbit each other. The rest of the time, the protostar collapses in isolation, as was the case for our Sun. In all cases, as we saw, conservation of angular momentum results in a spin-up of the collapsing protostar, with surrounding material flattened into a disk. Today, this kind of structure can actually be observed. The Hubble Space Telescope, as well as powerful new ground-based telescopes, enable astronomers to study directly the nearest of these circumstellar disks in regions of space where stars are being born today, such as the Orion Nebula (Figure 14.14) or the Taurus star-forming region.

Figure 14.14 : Protoplanetary Disk in the Orion Nebula. The Hubble Space Telescope imaged this protoplanetary disk in the Orion Nebula, a region of active star formation, using two different filters. The disk, about 17 times the size of our solar system, is in an edge-on orientation to us, and the newly formed star is shining at the center of the flattened dust cloud. The dark areas indicate absorption, not an absence of material. In the left image we see the light of the nebula and the dark cloud; in the right image, a special filter was used to block the light of the background nebula. You can see gas above and below the disk set to glow by the light of the newborn star hidden by the disk. (credit: modification of work by Mark McCaughrean (Max-Planck-Institute for Astronomy), C. Robert O’Dell (Rice University), and NASA)

Many of the circumstellar disks we have discovered show internal structure. The disks appear to be donut-shaped, with gaps close to the star. Such gaps indicate that the gas and dust in the disk have already collapsed to form large planets (Figure 14.15). The newly born protoplanets are too small and faint to be seen directly, but the depletion of raw materials in the gaps hints at the presence of something invisible in the inner part of the circumstellar disk—and that something is almost certainly one or more planets. Theoretical models of planet formation, like the one seen at right in Figure 14.15, have long supported the idea that planets would clear gaps as they form in disks.

Figure 14.15 : Protoplanetary Disk around HL Tau. (a) This image of a protoplanetary disk around HL Tau was taken with the Atacama Large Millimeter/submillimeter Array (ALMA), which allows astronomers to construct radio images that rival those taken with visible light. (b) Newly formed planets that orbit the central star clear out dust lanes in their paths, just as our theoretical models predict. This computer simulation shows the empty lane and spiral density waves that result as a giant planet is forming within the disk. The planet is not shown to scale. (credit a: modification of work by ALMA (ESO/NAOJ/NRAO); credit b: modification of work by NASA/ESA and A. Feild (STScI))

Our figure shows HL Tau, a one-million-year-old “newborn” star in the Taurus star-forming region. The star is embedded in a shroud of dust and gas that obscures our visible-light view of a circumstellar disk around the star. In 2014 astronomers obtained a dramatic view of the HL Tau circumstellar disk using millimeter waves, which pierce the cocoon of dust around the star, showing dust lanes being carved out by several newly formed protoplanets. As the mass of the protoplanets increases, they travel in their orbits at speeds that are faster than the dust and gas in the circumstellar disk. As the protoplanets plow through the disk, their gravitational reach begins to exceed their cross-sectional area, and they become very efficient at sweeping up material and growing until they clear a gap in the disk. The image of Figure 14.15 shows us that a number of protoplanets are forming in the disk and that they were able to form faster than our earlier ideas had suggested—all in the first million years of star formation.

Link to Learning

For an explanation of ALMA’s ground-breaking observations of HL Tau and what they reveal about plant formation, watch this videocast from the European Southern Observatory.

Discovering Exoplanets

You might think that with the advanced telescopes and detectors astronomers have today, they could directly image planets around nearby stars (which we call exoplanets). This has proved extremely difficult, however, not only because the exoplanets are faint, but also because they are generally lost in the brilliant glare of the star they orbit. As we discuss in more detail in The Birth of Stars and the Discovery of Planets outside the Solar System, the detection techniques that work best are indirect: they observe the effects of the planet on the star it orbits, rather than seeing the planet itself.

The first technique that yielded many planet detections is very high-resolution stellar spectroscopy. The Doppler effect lets astronomers measure the star’s radial velocity: that is, the speed of the star, toward us or away from us, relative to the observer. If there is a massive planet in orbit around the star, the gravity of the planet causes the star to wobble, changing its radial velocity by a small but detectable amount. The distance of the star does not matter, as long as it is bright enough for us to take very high quality spectra.

Measurements of the variation in the star’s radial velocity as the planet goes around the star can tell us the mass and orbital period of the planet. If there are several planets present, their effects on the radial velocity can be disentangled, so the entire planetary system can be deciphered—as long as the planets are massive enough to produce a measureable Doppler effect. This detection technique is most sensitive to large planets orbiting close to the star, since these produce the greatest wobble in their stars. It has been used on large ground-based telescopes to detect hundreds of planets, including one around Proxima Centauri, the nearest star to the Sun.

The second indirect technique is based on the slight dimming of a star when one of its planets transits, or crosses over the face of the star, as seen from Earth. Astronomers do not see the planet, but only detect its presence from careful measurements of a change in the brightness of the star over long periods of time. If the slight dips in brightness repeat at regular intervals, we can determine the orbital period of the planet. From the amount of starlight obscured, we can measure the planet’s size.

While some transits have been measured from Earth, large-scale application of this transit technique requires a telescope in space, above the atmosphere and its distortions of the star images. It has been most successfully applied from the NASA Kepler space observatory, which was built for the sole purpose of “staring” for 5 years at a single part of the sky, continuously monitoring the light from more than 150,000 stars. The primary goal of Kepler was to determine the frequency of occurrence of exoplanets of different sizes around different classes of stars. Like the Doppler technique, the transit observations favor discovery of large planets and short-period orbits. The Kepler observations closed down in 2018 when the spacecraft ran out of the fuel needed to point the telescope.

Recent detection of exoplanets using both the Doppler and transit techniques has been incredibly successful. Within two decades, we went from no knowledge of other planetary systems to a catalog of thousands of exoplanets. Most of the exoplanets found so far are more massive than or larger in size than Earth. It is not that Earth analogs do not exist. Rather, the shortage of small rocky planets is an observational bias: smaller planets are more difficult to detect.

Analyses of the data to correct for such biases or selection effects indicate that small planets (like the terrestrial planets in our system) are actually much more common than giant planets. Also relatively common are “super Earths,” planets with two to ten times the mass of our planet (Figure 14.16). We don’t have any of these in our solar system, but nature seems to have no trouble making them elsewhere. Overall, the Kepler data suggest that approximately one quarter of stars have exoplanet systems, implying the existence of at least 50 billion planets in our Galaxy alone.

Figure 14.16 : Transiting Planets by Size. This bar graph shows the planets found so far using the transit method (the vast majority found by the Kepler mission). The orange parts of each bar indicate the planets announced by the Kepler team in May 2016. Note that the largest number of planets found so far are in two categories that we don’t have in our own solar system—planets whose size is between Earth’s and Neptune’s. (credit: modification of work by NASA)

Discovery of Exoplanets

Dr. Geoff Marcy, UCSF-UCB (University of California San Francisco – University of California Berkeley) led the research efforts in the late 1980s to develop extrasolar planet search techniques. The main question was: What do you look for when trying to see a planet around another star?

Image courtesy of ESO/A. -M. Lagrange.

The image above shows an exoplanet caught on the move. Beta Pictoris is blocked from view because it is too bright to see it and the planet Beta Pictoris b at the same time.

Several techniques have been developed:

1. Technology has allowed both amateur and professional astronomers the ability to make such observations from here on Earth. And eventually technology should allow astronomers to directly view exoplanets.

   The Discovery of Extrasolar Planets

   As of fall 2014, astronomers have found 1,822 planets in 1,137 planetary systems and 467 multiple planet systems. Seven planets have been discovered around the star HD10180, the largest known system outside our solar system. And currently there are thousands of candidate exoplanets.

   So what are these exoplanets like?

   Most are much bigger than Earth, more like Jupiter and even much larger than Jupiter. Many are extremely hot; they orbit quite close to their star or stars. And many of these will “lose” to their parent stars and crash into their star as the star’s gravity pulls them in.

The Kepler Mission

The image above shows a size comparison of the exoplanets Kepler-20e and Kepler 20f with Venus and Earth. Image courtesy of NASA/Ames/JPL-Caltech.

The Kepler Mission is NASA’s first mission capable of finding Earth-size and smaller planets around other stars using an orbiting space telescope. The Kepler Space Telescope has been monitoring 100,000 stars similar to our Sun since its launch in March 2009. The Kepler Space Telescope, named for astronomer Johannes Kepler, looks for planets orbiting stars by looking for changes in a star’s brightness.

Kepler’s primary mission is to find Earth-size exoplanets. Detecting the massive gas giant exoplanets – called Super Jupiters – has been relatively easy with other methods. So the Kepler Space Telescope watches transits and measures the reduction in the star’s light due to the planet transiting in front of its star.

This image shows a size comparison of the exoplanets Kepler-20e and Kepler 20f with Venus and Earth.

Attributions

https://phys.libretexts.org/Bookshel...asolar_Planets

https://phys.libretexts.org/Bookshel...Kepler_Mission

https://phys.libretexts.org/Bookshel...netary_Systems