飞马座V342

HR 8799是一颗位于飞马座，距离地球129光年（39秒差距）的年轻（～6,000万年）主序星，质量大约是太阳的1.5倍，光度约为4.9倍。这个系统包含了部分的岩屑盘和至少3颗大质量行星（与北落师门 b相同，是第一批轨道以直接影像被证实的系外行星）。HR 8799是耶鲁亮星星表所使用的标识与编号。这颗星是剑鱼γ型变星：它的光度改变是表面非径向上的脉动造成的；这颗星也是牧夫座λ型星，这意味着它的表层是被耗尽铁峰顶元素，这也许可以归咎于金属的吸积缺乏拱星气体（星周气体）。它是唯一已知同时是剑鱼座γ型变星、牧夫座λ型星和类织女星（因为拱星盘而造成红外线过量）的恒星。

光谱类型和金属性

恒星HR 8799是分类为牧夫座λ型星的成员之一，这是在大气上层金属异常丰盈的一类特殊恒星族群。因为这种特别的状态，HR 8799有着非常复杂的光谱，与明锐的氢线和恒星的有效温度最吻合的光谱类型是F0 V，但是强烈的钙II K线和金属线却更像A5 V，因此它的光谱被标示为牧夫座kA5 hF0 mA5 V;。

对这颗恒星光谱的详细分析显示它的碳和氧与太阳比较有轻微的过量（大约各是30%和10%）。有一些牧夫座λ型星的硫有着与太阳相似的丰度，但是HR 8799没有这种情形，它的硫丰度只有太阳的35%。它同样缺乏比钠更重的元素，例如铁丰度只有太阳的28%。对牧夫座λ星震的观察建议这些怪异的丰度样式只被限制在表面：大部分的组成可能都是正常的。这也许暗示观测到的元素丰度只是因吸积而环绕在恒星周围的气体是缺乏金属的。

行星系统

在2008年11月13日，加拿大赫兹伯格天体物理学院的克莉斯汀马洛斯宣布他们的团队使用在夏威夷的凯克天文台和双子星天文台的望远镜直接观测到3颗环绕着HR 8799的系外行星，在这两个情况下是使用了自适应光学以红外线观测。稍后，在使用新的影像处理技术后，从新回顾哈勃空间望远镜在1998年使用NICMOS拍摄的影像中也找到了最外围的那颗系外行星。最外围的那颗行星正好位于像太阳柯伊伯带一样的灰尘盘。它是在距离地球300光年的范围内环绕恒星的盘面中最重的一个，并且有类地行星在内的系统。

行星d、c和b的轨道半径分别是相对于土星、天王星和海王星的2至2.5倍，因为平方反比定律联系起辐射密度与来源的距离，可以比较辐射强度与距离的关系为 = 2.2，即与HR 8799的距离是太阳系内的2.2倍，意味着在HR 8799系统内的行星接受到的辐射数量是与太阳系的行星相当的。

这些天体的质量已经接近行星质量的上限，如果他们超过木星质量的13倍，他们就足以引发在核心的氘进行核聚变，依据国际天文联合会的系外行星工作小组的定义，就将成为合格的褐矮星。如果对质量的估计是正确的，HR 8799的系统就会是第一个被直接观察到的多星系外行星系。通过与2004年的观测比较，证实了这些行星的运动是逆时针方向，发现的论文同时建议轨道是接近圆形并且几乎是以正面朝向地球，动力学的模拟要求这些行星必须有比目前假设更低的质量，否则系统在比目前恒星年龄更短的时间尺度上是不稳定的。如果在内侧的两颗行星有2:1的共振，系统会更为稳定，这也暗示了最内侧行星的离心率会超过0.04的程度，以匹配观测上的限制。如果这些行星有1:2:4的轨道共振（类似于木星内侧3颗伽利略卫星：艾奥、欧罗巴和盖尼米得的拉普拉斯共振），在演化模型上行星系统将会有最稳定的质量。 如果证实了，HR 8799的行星系统将是第一个被观测的多重共振系外行星系。这样的系统即使将质量增加至目前的1.9倍，依然能够保持稳定的状态。

HR 8799 is a roughly 42 million-year-old main-sequence star located 133.3 light-years (40.9 parsecs) away from Earth in the constellation of Pegasus. It has roughly 1.4 times the Sun's mass and 5 times its luminosity. It is part of a system that also contains a debris disk and at least four massive planets. These planets were the first exoplanets whose orbital motion was confirmed by direct imaging. The star is a Gamma Doradus variable: its luminosity changes because of non-radial pulsations of its surface. The star is also classified as a Lambda Boötis star, which means its surface layers are…

Location

HR 8799 is a star that is visible to the naked eye with sufficiently dark skies. It has a magnitude of about 6.0, varying by a few hundredths of a magnitude, and it is located inside the western edge of the great square of Pegasus almost exactly halfway between Beta and Alpha Pegasi.

The star's name of HR 8799 is its number in the Bright Star Catalogue. It also has the variable star designation V342 Pegasi.

Stellar properties

The star HR 8799 is a member of the Lambda Boötis (λ Boo) class, a group of peculiar stars with an unusual lack of "metals" (elements heavier than hydrogen and helium) in their upper atmosphere. Because of this special status, stars like HR 8799 have a very complex spectral type. The luminosity profile of the Balmer lines in the star's spectrum, as well as the star's effective temperature, best match the typical properties of an F0 V star. However, the strength of the calcium II K absorption line and the other metallic lines are more like those of an A5 V star. The star's spectral type is ther…

Age determination of this star shows significant variation depending on the method used. Statistically, for stars hosting a debris disk, the luminosity of this star suggests an age of about 20–150 million years. Comparison with stars having similar motion through space gives an age in the range 30–160 million years. Given the star's position on the Hertzsprung–Russell diagram of luminosity versus temperature, it has an estimated age in the range of 30–1,128 million years. λ Boötis stars like this are generally young, with a mean age of a billion years. More accurately, asteroseismology suggest…

Earlier analysis of the star's spectrum reveals that it has a slight overabundance of carbon and oxygen compared to the Sun (by approximately 30% and 10% respectively). While some Lambda Boötis stars have sulfur abundances similar to that of the Sun, this is not the case for HR 8799; the sulfur abundance is only around 35% of the solar level. The star is also poor in elements heavier than sodium: for example, the iron abundance is only 28% of the solar iron abundance. Asteroseismic observations of other pulsating Lambda Boötis stars suggest that the peculiar abundance patterns of these stars a…

In 2020, spectral analysis using multiple data sources have detected an inconsistency in prior data and concluded the star carbon and oxygen abundances are the same or slightly higher than solar. The iron abundance was updated to 30⁺⁶ −5% of the solar value.

A 2011 asteroseismic analysis using spectroscopic data suggested that the rotational inclination of the star is greater than or approximately equal to 40°. However, such measurements are difficult due to the complex nature of the pulsations. If this value is accurate, it would contrast with the planets' orbital inclinations, which are in roughly the same plane with measured values between 22° and 28°. Hence, there may be an unexplained misalignment between the rotation of the star and the orbits of its planets. A more recent photometric analysis suggests a rotational inclination of 28+14 −3°, …

Planetary system

On 13 November 2008, Christian Marois of the National Research Council of Canada's Herzberg Institute of Astrophysics and his team announced they had directly observed three planets orbiting the star with the Keck and Gemini telescopes in Hawaii, in both cases employing adaptive optics to make observations in the infrared. A precovery observation of the outer 3 planets was later found in infrared images obtained in 1998 by the Hubble Space Telescope's NICMOS instrument, after a newly developed image-processing technique was applied. Further observations in 2009–2010 revealed the fourth giant p…

HR 8799 b orbits inside a dusty disk like the Solar Kuiper belt. It is one of the most massive disks known around any star within 300 light years of Earth, and there is room in the inner system for terrestrial planets. There is an additional debris disk just inside the orbit of the innermost planet.

The orbital radii of planets e, d, c, and b are 2–3 times those of Jupiter, Saturn, Uranus, and Neptune's orbits, respectively. Because of the inverse square law relating radiation intensity to distance from the source, comparable radiation intensities are present at distances √4.9 ≈ 2.2 times farther from HR 8799 than from the Sun, it serendipitously turns out that corresponding planets in the solar and HR 8799 systems receive similar amounts of stellar radiation.

HR 8799 system is the first multiple-planet extrasolar system to be directly imaged. The orbital motion of the planets is in an anticlockwise direction and was confirmed via multiple observations dating back to 1998. The system is more likely to be stable if the planets e, d, and c are in a 4:2:1 resonance, which would imply that the orbit of the planet d has an eccentricity exceeding 0.04 in order to match the observational constraints. Planetary systems with the best-fit masses from evolutionary models would be stable if the outer three planets are in a 1:2:4 orbital resonance (similar to th…

The broadband photometry of planets b, c and d has shown that there may be significant clouds in their atmospheres, while the infrared spectroscopy of planets b and c points to non-equilibrium CO / CH₄ chemistry. Near-infrared observations with the Project 1640 integral field spectrograph on the Palomar Observatory have shown that compositions between the four planets vary significantly. This is a surprise since the planets presumably formed in the same way from the same disk and have similar luminosities.

Additional planets

Since the discovery of planet e, many astronomers have speculated about the presence of a fifth planet in the system. A fifth inner planet could continue the chain of resonances if it were at a 3:1 or 2:1 resonance with HR 8799 e. Direct imaging has not yet been able to detect such an object due to low planet-to-star luminosity ratio and short angular separation, deriving upper limits for the presence of additional planets. The tightest limits as of 2021 rule out a planet with a mass over 3.6 MJ at 7.5 au, and over 2.8 MJ at 9.7 au.