Theme 7 The Complete Solar System; Other Planetary Systems

Transcription

1 Theme 7 The Complete Solar System; Other Planetary Systems ASTR 101 Prof. Dave Hanes

2 After the Formation of the Planets We might expect to see in the Solar System: 1. leftover gas 2. evidence of large collisions that took place as the biggest lumps merged 3. leftover rubble dating back to the formation stage (unaccreted lumps) - plus evidence that it all happened at about the same time

3 Around Other Stars we might also expect to see evidence of such processes, producing much the same results. These could be in the form of either: n gaseous disks (planets forming right now) or n established planetary systems (from long ago).

4 But We Must Aware of the possibility that some things may have significantly changed (either in the SS, or in other systems) since the formative stages. Some important evidence may have been eradicated; or the final arrangement may have slowly evolved over billions of years

5 1. Leftover Gas? We do NOT see this: Instead, the planets travel through mostly empty space, so any leftover gas is long gone. But H and He could not have condensed in the inner Solar System. Where did that gas go?

6 T Tauri Stars Young stars of about the same mass as the Sun go through an early stage where they have enormously strong stellar winds consisting of charged particles. This can sweep out all the leftover gas (and even scour off much of the Earth s primitive atmosphere!).

7 Earth s Atmosphere It is secondary, created somewhat later by n outgassing from volcanoes n gradual accumulation from the vaporization of incoming grains, pebbles, meteors, comets (rich in icy material), etc (It is even possible that much of the Earth s water was carried in by comets!)

8 It Has Also Evolved Subsequently Note the decrease in Carbon Dioxide with the emergence of plant life and the growth in the abundance of Oxygen!

9 2. Evidence of Major Collisions On Earth: our active geology and weather quickly erodes away any evidence of impacts. Not very helpful! Moon Mercury Vesta (asteroid) But on older airless surfaces (the Moon, Mercury, other small bodies): the bombardment history is recorded in the distribution, size and number of impact craters.

10 The Record on the Moon Evidence of very heavy impact rates ~4.5 billion years ago, then a brief period of late heavy bombardment about 3.9 b.y. ago that has since levelled out.

11 Some Really Big Impacts Major impacts may have been responsible for n n the retrograde (backwards) spin of Venus the fact that Uranus is tipped on its side as it spins and possibly also the formation of our Moon!

12 The Formation of the Moon - some very strong evidence for this, but not yet compelling

13 3. Leftover Rubble in the SS The Solar System is full of meteoroids, comets, asteroids, small objects out beyond the orbit of Pluto, and so on. That is, not every chunk wound up being accreted by a growing planet! We return to this topic in a later unit.

14 When Did This All Happen? In the next section, we will discuss age-dating techniques and the age of the Solar System.

15 Other Planetary Systems? First, do we see evidence of proto-planetary disks around nearby stars? Remember than this distributed material will be cool, not glowing in the visible. So we use infrared (or even radio radiation) to hunt for such disks.

16 Yes! The Star Beta Pictoris as seen in 1983 by IRAS (the Infrared Astronomical Satellite) (the light from the bright central star has been blocked off here)

17 Much Better Technology Now The star HL Tau (with an estimated age of about 1 million years) as seen in 2015 by the new ALMA telescope in Chile Notice the disk of cool material (gas and dust) with gaps where protoplanets are sweeping up material.

18 Established Exo-Planetary Systems A reminder of various detection techniques: n Get direct images of planets as dots of light; this is very hard next to the bright parent star! Now possible for a few. n As a planet orbits, we detect the wobbling of the star (a) towards and away from us measurable in the spectral lines, using the Doppler shift (b) from side to side not yet possible n As a planet passes briefly in front of the star (a transit), we see a repeated periodic dimming of the light

19 Direct Imaging? Hard! The star is bright; a planet is faint (Imagine a firefly beside a searchlight!) Three planets are seen here. (The light of the star is masked.)

20 Detecting Velocity Wobbles [the first discovery of exoplanets] A reminder of why this happens:

21 Transits Can Be Studied Better data from space!

22 The Kepler Space Telescope Detected Many!

23 Recent Interesting Findings

24 Beware Strong Selection Effects Planets that are big in size are easiest to find: they block off more light during transits Planets that are large in mass are easiest to find: they make the parent star wobble more in velocity Planets that are closer to the star are easiest to find: they are more likely to produce a transit, and have stronger gravitational ( wobbling ) effects

25 This Explains Why in many of the planetary systems detected so far, we find big, massive planets quite close to the parent stars (especially with the wobble technique; using transits is better able to find smaller planets.) It will take many years, and improving technology, to allow the confirmed detection of a Solar System like our own.

26 Lots of Big Planets Here, overall discoveries. Notice the bias in favour of big planets The Upsilon Andromedae system has three Jupiterlike planets very close to the star!

27 One Puzzling Aspect Finding big planets is not so surprising, but why are so many Jupiters so close to the parent stars? In the nebular model, only small rocky planets should form in that region. (Only a fraction of the initial material can condense where it is hot, so the inner planets shouldn t be huge.)

28 A New Understanding Complex gravitational interactions between planets can cause them to migrate (move around in the planetary system) over the passage of many millions of years. Smaller planets may even be ejected from the system entirely.

29 Lucky Us! Computer models suggest that the big outer planets in our own Solar System may indeed have migrated to some extent, with important effects on the orbits of the asteroids, the many small objects beyond Neptune, and Uranus and Neptune themselves. But by good fortune the Earth s orbit has been relatively stable, and life on Earth has survived. Other planetary systems may not be so fortunate

30 One Goal Look for signs of life, like a planetary atmosphere rich in oxygen (as indicated by its spectrum)

31 To Find Out More The James Webb Space Telescope has set this detection as one of its aims.