Superflares and Giant Planets

Superflares and Giant Planets From time to time, a few sunlike stars produce gargantuan outbursts. Large planets in tight orbits might account for these eruptions Eric P. Rubenstein nvision a pale blue planet, not un- bushes to burst into flames. Nor will the lar flares, which typically last a fraction Elike the Earth, orbiting a yellow star surface of the planet feel the blast of ul- of an hour and release their energy in a in some distant corner of the Galaxy. traviolet light and x rays, which will be combination of charged particles, ul- This exercise need not challenge the absorbed high in the atmosphere. But traviolet light and x rays. Thankfully, imagination. After all, astronomers the more energetic component of these this radiation does not reach danger- have now uncovered some 50 “extra- x rays and the charged particles that fol- ous levels at the surface of the Earth: solar” planets (albeit giant ones). Now low them are going to create havoc The terrestrial magnetic field easily de- suppose for a moment something less when they strike air molecules and trig- flects the charged particles, the upper likely: that this planet teems with life ger the production of nitrogen oxides, atmosphere screens out the x rays, and and is, perhaps, populated by intelli- which rapidly destroy ozone. the stratospheric ozone layer absorbs gent beings, ones who enjoy looking So in the space of a few days the pro- most of the ultraviolet light. So solar up at the sky from time to time. tective blanket of ozone around this flares, even the largest ones, normally During the day, these creatures planet will largely disintegrate, allow- pass uneventfully. People living at high would see their sun shining brightly, ing ultraviolet light to rain down latitudes may see unusually intense providing them with an almost constant unimpeded. Until this shield is re- auroral displays. But unless an espe- bath of warming rays. At night they stored, a process that might take cially strong flare knocks out a com- might gaze at a moon or two against a decades, daylight will remain deadly munication satellite or a power grid, backdrop of seemingly unchanging to the organisms exposed to it. few people take much notice. stars. Over the billions of years since life Fortunately, our Sun does not ap- first evolved on this hypothetical planet, pear prone to such disastrous explo- Inconstant Suns this world has been spared any cosmic sions. Indeed, it produces a remarkably Not all stars shine as steadily as the catastrophes. No nearby supernovae steady output of energy—the value of Sun. Some change in brightness over a have exploded. The orbit of the planet which is duly named the solar con- period of hours, days or months. As- has stayed circular. And no great aster- stant. Sensitive instruments placed on tronomers are fascinated by such vari- oids have come crashing down. In satellites show that it varies by only a able stars, and groups of observers have short, none of the many celestial events few parts in a thousand. Astrophysi- organized specifically to examine that would threaten life here have hap- cists do, however, surmise that the so- them. Fortunately for those of us inter- pened. But today is a bad day. lar constant is slowly increasing, by ested in such curious stars, there is an A stellar flare, enormous by normal about one percent each 300 million uncountable number of them. standards, erupts from the surface of years. Over the 4.6 billion years since Astronomers classify these variable the parent star, giving off a sudden the solar system formed, the Sun has stars according to their properties. So- burst of radiation and spewing into brightened by about 40 percent. Al- called geometric variables do not actually space a fusillade of charged particles. though this variation is substantial, it change the amount of energy they give Although it will startle the inhabitants, has taken place so gradually that off. Rather, the geometry of viewing the sudden heat wave will not cause changes in the amount of heat-trap- these multiple star systems causes us ping gases in the atmosphere have to see different amounts of light at dif- compensated, allowing the tempera- ferent times. In most cases the varia- Eric P. Rubenstein received a Ph.D. in astronomy ture of the Earth to remain relatively tion arises because one star in a binary from Yale University in 1997. He remained there steady—and in a range that keeps wa- system passes in front of the other. for two years before becoming a National Science ter liquid—for billions of years. Such eclipses are most likely to be seen Foundation International Research Fellow and Life on Earth clearly benefits from if the two stars revolve around each starting work at the Cerro Tololo Inter-American Observatory in Chile. Last summer he returned to this long-term stability and from the other in a tight orbit. Interestingly, such Yale as a lecturer in the Department of Astronomy. Sun having only moderate levels of binaries, ones with small separations Address: Yale University, Department of Astrono- what astronomers refer to as “activity,” between the stars, commonly exhibit my, P.O. Box 208101, New Haven, CT 06520- meaning violent outbursts. In large additional kinds of variability. Tidal in- 8101. Internet: [email protected] part, this activity manifests itself as so- teractions cause these closely spaced 38 American Scientist, Volume 89 Copyright © 2001 American Scientist Figure 1. Artistic rendition of a distant, sunlike star undergoing a “superflare” hints at the enormity of such an event. These huge eruptions dwarf the largest flares on the Sun, releasing from 100 to 10 million times as much energy. The author proposes that giant planets orbiting close to such stars trigger these occasional outpourings. (Image courtesy of the author.) Copyright © 2001 American Scientist 2001 January–February 39 stars to spin rapidly, which in turn galaxy. Other examples of this class in- range from several seconds to hun- greatly enhances their magnetic fields, clude binary star systems in which a dreds of days. Some vary in brightness causing strong stellar flares. As- compact white dwarf pulls in hydrogen by more than an order of magnitude. tronomers call such flaring chromos- from its nearby partner. From time to Astrophysicists have determined that pheric activity, because the energy time the rain of gas gives rise to huge such changes result from instabilities comes from the portion of the stellar thermonuclear explosions on the sur- in either the rate of energy generation atmosphere that also gives the star its face of the white dwarf. Such systems, or in the relationship between temper- characteristic color. dubbed cataclysmic variables, typically ature and opacity in the stellar atmos- Even more spectacular are the erup- experience repeated explosions, so the phere. Pulsators are usually periodic, tive variables, which include super- term “cataclysmic” is not really apt. although some have multiple period- novae—stars that end their lives by ex- Skywatchers also recognize pulsating icities superposed. ploding so violently that they can variables, which alternately increase So variable stars are themselves a outshine all the other stars in their and decrease in size over intervals that varied lot—and a well-studied one. In- deed, astronomers have prided them- 1 Algol selves in their understanding of the mechanisms that bring about these changes. Conversely, we generally be- lieved that we could predict which stars would be as stable as the Sun. We have traditionally assumed, for instance, that if a star has roughly the same surface temperature and luminosity as the Sun, relative brightness relative is a single star and rotates at a speed similar to that of the Sun, it will likewise 0 023451 have only modest levels of chromos- days pheric activity. Such stars are commonly called solar analogues. The unspoken as- 1 SS Cygni sumption that all solar analogues are, in essence, interchangeable underlies much of the thinking about habitable worlds, and perhaps life, existing else- where in the cosmos. Although it seems reasonable that solar analogues should be as stable as the Sun, it is clear that this assumption relative brightness relative was premature—and wrong. Astron- omers now know of nine instances of 0 0 24 6810 12 these sunlike stars having eruptions months that were from 100 to 10 million times 1.5 more energetic than the most powerful 1 Sun per square meter) (kilowatts solar flares. In these cases the stars tem- brightness porarily brightened by amounts that range from several percent up to a fac- Chi Cygni solar flux 1.0 solar flux tor of 10 or more. Startling as they would be for some- one living nearby, giant flares around 0.5 distant stars can easily elude us earth- lings. For the most part, the sightings relative brightness relative of these events were simply fortuitous. One set of observations was, for ex- 0 0 ample, made on March 6, 1899, when 1980 1985 1990 1995 2000 a newly discovered comet passed years through the constellation Fornax and entered the part of the sky that contains Figure 2. Many variable stars exhibit characteristic patterns of changing brightness, which a star called S Fornacis. Four experi- can reveal much about their nature. The star Algol (Beta Persei), for example, shows a repeti- enced and highly respected observers tive pattern of short-lived diminutions in brightness (top). Astronomers regard Algol as a went out early that evening to view the “geometric variable” because the changes in brightness result from the viewing geometry, which allows each star in this binary system to periodically eclipse the other.

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