Heliophysics

We live in the extended atmosphere of an active star. While sunlight enables and sustains life, the Sun's variability produces streams of high energy particles and radiation that can harm life or alter its evolution.

Under the protective shield of a magnetic field and atmosphere, the Earth is an island in the Universe where life has developed and flourished. The origins and fate of life on Earth are intimately connected to the way the Earth responds to the Sun's variations.

Understanding the Sun, Heliosphere, and Planetary Environments as a single connected system is the goal of the Science Mission Directorate's Heliophysics Research Program. In addition to solar processes, our domain of study includes the interaction of solar plasma and radiation with Earth, the other planets, and the Galaxy. By analyzing the connections between the Sun, solar wind, planetary space environments, and our place in the Galaxy, we are uncovering the fundamental physical processes that occur throughout the Universe. Understanding the connections between the Sun and its planets will allow us to predict the impacts of solar variability on humans, technological systems, and even the presence of life itself.

We have already discovered ways to peer into the internal workings of the Sun and understand how the Earth's magnetosphere responds to solar activity. Our challenge now is to explore the full system of complex interactions that characterize the relationship of the Sun with the solar system. Understanding these connections is especially critical as we contemplate our destiny in the third millennium. Heliophysics is needed to facilitate the accelerated expansion of human experience beyond the confines of our Earthly home. Recent advances in technology allow us, for the first time, to realistically contemplate voyages beyond the solar system.

There are three primary objectives that define the multi-decadal studies needed:

• To understand the changing flow of energy and matter throughout the Sun, Heliosphere, and Planetary Environments.
• To explore the fundamental physical processes of space plasma systems.
• To define the origins and societal impacts of variability in the Earth-Sun System.

A combination of interrelated elements is used to achieve these objectives. They include complementary missions of various sizes; timely development of enabling and enhancing technologies; and acquisition of knowledge through research, analysis, theory, and modeling.

PLANETS AROUND OTHER STARS

What are exoplanets?

Throughout recorded history and perhaps before, we have wondered about the possible existence of other worlds, like or unlike our own. The earliest understanding of the solar system showed us that there were indeed other worlds in orbit about our Sun, and steadily growing understanding of their natures shows that all are dramatically different from Earth, and mostly very different from one another. As we came to understand that the stars in the sky are other suns, and that the galaxies consist of billions of stars, it appeared a near certainty that other planets must orbit other stars. And yet, it could not be proven, until the early 1990’s. Then, radio and optical astronomers detected small changes in stellar emission which revealed the presence of first a few, and now many, planetary systems around other stars. We call these planets “exoplanets” to distinguish them from our own solar system neighbors.

How we know that there are planets around other stars?

Most of the detected exoplanets have revealed their presence by small effects that they have on their star. As planet follows its orbital path, the star follows a complementary motion of its own. This is a tiny effect proportional to the planet/star mass ratio – in the case of the solar system, the Sun moves in synch with the Earth at the speed of a slow dance – currently too slow to readily detect in a distant system. The motion of the Sun in synch with Jupiter, however, is closer to a fast run – and in favorable cases it can be detected by several methods. The motion of the host star can be measured as a shift in its spectrum (the Doppler shift) or as a change in its position on the sky (astrometry). In both cases these are very challenging measurements and require exquisitely sensitive instruments. Exoplanet orbits presumably have random orientations, and in some cases the orbit carries the planet between us and its star. Then the exoplanet might be detected by the decrease in the light from the star. Such transitshave been observed, and a number of planets discovered by this method.

Another effect that can reveal the presence of a planet around another star is the bending of light from background stars by the gravitational field of an intervening star. If the intervening star has an orbiting planet it may alter the gravitational lensing effect in a noticeable way (microlensing). The large majority of the several hundred known extrasolar planets have been discovered by the Doppler technique, and other methods are contributing more significantly as they are refined and the number of detected exoplanets continues to increase steadily.

What do we know about our exoplanet neighbors?

Although the details are not entirely understood, it is known that stars like the Sun form from spinning protostellar disks of gas and dust. The Earth and other planets of the solar system are believed to have developed from the remains of that disk, and there is no reason to believe that the same process would not be effective throughout the galaxy. Thus a first guess might be that other planetary systems would be like the solar system.

However, the first detections of exoplanets revealed bodies which are utterly unlike any solar system planet – and subsequent discoveries have shown that many exoplanet systems are very dissimilar from ours. In some exosystems, planets as massive as Jupiter orbit so close to their star that they are heated to high temperature and their upper atmospheres are swept into space. In other systems, planets follow elongated orbits (in contrast to the nearly circular orbits of the solar system). However, our studies of exoplanets are just beginning, and it is not possible to be sure what will prove to be “typical” planets among our neighboring stars. Will most planet systems prove to be much like our own, or are we exceptional in more ways than we can imagine? Only years of further study will tell.

Evidence is accumulating that exoplanet systems which resemble the solar system are being found. The star 55 Cancri, 41 light years away, has a system of 5 planets, with distributions somewhat similar to the solar systems inner planets (though with much higher masses). As our measurements become sensitive to lower masses, some astronomers believe that we will find many such systems with a substantial complement of planets (perhaps even dynamically full – that is, containing as many planets as can coexist in orbital harmony).

In other reports, a number of planets with masses near that of Earth have been detected. The results are few, but because the measurements are very difficult, the detections are considered significant and possibly indicative of many more to be found in the future. Again, only years of study will tell.

What do we want to learn about exoplanets?

A thorough understanding of exoplanets will tell us much about how our solar system formed, why it has small, rocky planets near the Sun, why it has gas giant planets far from the Sun, why the Earth has the conditions and chemicals that can support life, and why conditions on other planets are hostile to life. Theories of planet formation and evolution are incomplete, but offer specific predictions. Detections of exoplanets are already testing, validating, and in some cases invalidating, details of these theories.

Perhaps the most interesting question, and one of the most difficult to answer, concerns the uniqueness of the Earth. Are there planets similar to the Earth around other stars and does life exist on any other planet beyond our own Earth?

Recent Discoveries

April 18, 2013	Kepler's Smallest Habitable Zone Planets (Kepler-62 and Kepler-69 systems)
April 5, 2013	Dead Star Warps Light of Companion Red Star, Astronomer Say
March 29, 2013	Planet Hunters Discover: PH1b (Kepler-64b)
February 20, 2013	Tiny Planet System (Kepler-37)
February 6, 2013	Earth-size Planet May Be Next Door
January 10, 2013	At Least One in Six Stars has an Earth-sized Planet
January 8, 2013	Two Belts and Possibility of Planets (Vega)
January 8, 2013	Rogue Planetary Orbit for Fomalhaut b
January 3, 2013	Billions and Billions of Planets
November 27, 2012	Solar System with Extra Comets (GJ 581 and 61 Vir)
November 1, 2012	Asteroid Belts of Just the Right Size are Friendly to Life
October 18, 2012	Revisiting Exoplanet TrES-2 (Kepler-1b)
October 15, 2012	Citizen Scientists Discover Four-Planets
September 11, 2012	Extreme Life Forms Might be Able to Survive on Eccentric Exoplanets
August 28, 2012	Sharing the Light of Two Suns (Kepler-47c)
August 22, 2012	41 New Transiting Planets in Kepler Field of View
July 18, 2012	Exoplanet is Extremely Hot and Incredibly Close
July 5, 2012	Mysterious Case of the Disappearing Dust
June 28, 2012	Hubble and Swift Detect First-Ever Changes in an Exoplanet Atmosphere
June 21, 2012	Astronomers Discover Planetary Odd Couple (Kepler-36b, Kepler-36c)

How did the universe originate and evolve to produce the galaxies, stars, and planets we see today?

How did we get here? In order to understand how the Universe has changed from its initial simple state following the Big Bang (only cooling elementary particles like protons and electrons) into the magnificent Universe we see as we look at the night sky, we must understand how stars, galaxies and planets are formed.

There are many questions associated with the creation and evolution of the major constituents of the cosmos. A basic question astronomers must address is, how did the Universe create its first stars and galaxies? Once these entities were created, how did they influence subsequent galaxy, star and planet formation? This is an important question, because these later objects are made of elements that can only have been created by the first generation of stars.

It is still unknown whether the Universe created black holes with the first generation of stars or whether these exotic objects were created by the first generation of stars. Because black holes represent the most extreme physical conditions of spacetime and generate some of the most energetic phenomena following the Big Bang, they are the ultimate physical laboratories for testing theories of the Universe.

We now know that our Universe has a "foamy" structure. The galaxies and clusters of galaxies that make up the visible Universe are concentrated in a complex scaffold that surrounds a network of enormous cosmic voids. However, in addition to the "normal" matter that makes up the visible parts of the Universe, scientists have discovered that there are vast amounts of unseen matter. This so-called, "dark matter" makes up roughly 23% of the matter-energy content of the Universe, while the visible pieces account for only about 5% of the total. Clearly, if we hope to understand the structure of the Universe and the processes by which it formed and evolves, we must first understand the distribution of this important but unseen dark matter and the ways in which it interacts with and influences normal matter.

Though astronomers have been studying stars for thousands of years, it is only in the past 35 or so years that they have been able to employ instruments that detect light across the entire electromagnetic spectrum–from radio waves to gamma rays–to peer into the dusty clouds where stars are born in our own Galaxy. If we are to comprehend how the Universe makes stars–and planets that orbit them today–we must continue these studies with ever more powerful telescopes.

Black holes

Don't let the name fool you: a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area - think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City. The result is a gravitational field so strong that nothing, not even light, can escape. In recent years, NASA instruments have painted a new picture of these strange objects that are, to many, the most fascinating objects in space.

Intense X-ray flares thought to be caused by a black hole devouring a star. (Video)

Although the term was not coined until 1967 by Princeton physicist John Wheeler, the idea of an object in space so massive and dense that light could not escape it has been around for centuries. Most famously, black holes were predicted by Einstein's theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core's mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole.

Using radio telescopes located throughout the Southern Hemisphere scientists have produced the most detailed image of particle jets erupting from a supermassive black hole in a nearby galaxy. (Video)

Scientists can't directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation. We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby. If a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion. A similar process can occur if a normal star passes close to a black hole. In this case, the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them - emitting powerful gamma ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others.

Astronomers have identified a candidate for the smallest-known black hole. (Video)

One Star's End is a Black Hole's Beginning

Most black holes form from the remnants of a large star that dies in a supernova explosion. (Smaller stars become dense neutron stars, which are not massive enough to trap light.) If the total mass of the star is large enough (about three times the mass of the Sun), it can be proven theoretically that no force can keep the star from collapsing under the influence of gravity. However, as the star collapses, a strange thing occurs. As the surface of the star nears an imaginary surface called the "event horizon," time on the star slows relative to the time kept by observers far away. When the surface reaches the event horizon, time stands still, and the star can collapse no more - it is a frozen collapsing object.

Even bigger black holes can result from stellar collisions. Soon after its launch in December 2004, NASA's Swift telescope observed the powerful, fleeting flashes of light known as gamma ray bursts. Chandra and NASA's Hubble Space Telescope later collected data from the event's "afterglow," and together the observations led astronomers to conclude that the powerful explosions can result when a black hole and a neutron star collide, producing another black hole.

Babies and Giants

Although the basic formation process is understood, one perennial mystery in the science of black holes is that they appear to exist on two radically different size scales. On the one end, there are the countless black holes that are the remnants of massive stars. Peppered throughout the Universe, these "stellar mass" black holes are generally 10 to 24 times as massive as the Sun. Astronomers spot them when another star draws near enough for some of the matter surrounding it to be snared by the black hole's gravity, churning out x-rays in the process. Most stellar black holes, however, lead isolated lives and are impossible to detect. Judging from the number of stars large enough to produce such black holes, however, scientists estimate that there are as many as ten million to a billion such black holes in the Milky Way alone.

On the other end of the size spectrum are the giants known as "supermassive" black holes, which are millions, if not billions, of times as massive as the Sun. Astronomers believe that supermassive black holes lie at the center of virtually all large galaxies, even our own Milky Way. Astronomers can detect them by watching for their effects on nearby stars and gas.

Astronomers may have found evidence for a cluster of young, blue stars encircling one of the first intermediate-mass black holes ever discovered.Read the full article

Historically, astronomers have long believed that no mid-sized black holes exist.  However, recent evidence evidence from Chandra, XMM-Newton and Hubble strengthens the case that mid-size black holes do exist. One possible mechanism for the formation of supermassive black holes involves a chain reaction of collisions of stars in compact star clusters that results in the buildup of extremely massive stars, which then collapse to form intermediate-mass black holes. The star clusters then sink to the center of the galaxy, where the intermediate-mass black holes merge to form a supermassive black hole.